From the Zimbra forums:

LetushostU is currently planning our migration to the new version. A timeline for this will be posted soon. Informal estimate is 2-3 weeks depending on any major bugs being found in the point-0 release.

Here’s a quick list of new features to expect. A more detailed analysis will be forthcoming.

Zimbra Web Client Features

•        Calendar. The attendee can select from a drop down menu to not notify the creator when they accept, decline, or tentatively accept a meeting. (Bug 38179)
•        Calendar. A vertical stripe on an appointment indicates that status. Those colors represented as follows: yellow as out-of-office, white as free, orange as tentative, and brown as accepted. (Bug 13184)
•        The Preferences tab-view has been moved to the Navigation pane and displays as folders. The Signatures, Accounts, and Filters folders display in the Mail folder. New folders are Sharing, Mobile Devices, and Zimlets. (Bug 35589)
•        A new Preferences>Sharing folder lets users see a list of folders shared with them and folders they share with others. (Bug 7473)
•        A new Preferences>Zimlets folder lets users manage the Zimlets that the system administrator enabled for their accounts. (Bug 12383)
•        Users can run filter rules over existing sets of mail. They select the filters to run against existing mail and click the Run Filter button on the toolbar. (Bug 1112)
•        Mandatory spell check of email messages can be enabled from the Preferences>Composing folder. (Bug 5460)ZCS 6.0
•        Users can set up their own Junk Mail Options of whitelist and blacklist email addresses that is used to filter incoming messages. This can be done from the Preferences>Mail folder. The default maximum number of whitelist and black list addresses is 100 for each. This value can be changed from zmprov account or COS attributes zimbraMailWhitelistMaxNumEntries and zimbraMailBlacklistMaxNumEntries. (Bug 6953)
•        Calendar – Users can view their calendars in a list view. In the list view, users can:
•        See all the appointments in selected calendars for a two week period
•        Select multiple appointments to act upon at once.
•        Delete or move appointments that are on local calendars. If an appointment is recurring, they delete all appointments in the series.
•        Tag appointments for local calendars (Bug 19338)
•        When a user has admin rights on a shared Calendar and an invite is sent, a copy of the meeting invite is forwarded to the admin-user’s mailbox. The admin-user can accept/decline the invite from their own Inbox.(Bug 23336)
•        Users can set up the email addresses to receive a copy of their invitations from their Preferences>Calendar folder. This is a multi-valued attribute. (Bug 36480)
•        Users can now configure whether new appointments are added to their calendar automatically before they reply to the invite or added only when they select Accept or Tentative in the message. The default behavior is to have all appointments display in the calendar unless deleted. This preference is set from the Preferences>Calendar folder in the Automatically add received appointments to calendar field.
Once users accept a meeting, any updates, including cancellations sent for the same meeting are auto-applied to the users calendar without approval in all options. (Bug 30990 36180)
•        Users can configure their Calendar Preferences to automatically decline invites from an organizer who does not have permission to invite the receiving user. This preference is set from the Preferences>Calendar folder, Permissions, Invites section. (Bug 30990 36180)
Users can manage whether to send a return receipt when a message is requesting a read receipt, from their Preferences>Mail folder, Read Receipt section. (Bug 7257)
•        Search. If you know where to search for an item, you can do a quick search by entering one of the search keywords followed by a colon (tag:) in the search field. The keywords are tag, in, under, is, has, type, attachment. (Bug 37737)
•        Added the ability to sort by message size (Bug 10445)
•        Composing a message and opening a single message now opens a new tab. Multiple tabs can be open. Users can move between tabs. When they send or cancel a composed message, the compose tab is closed. (Bug 9585)
•        Users can select to view their mailbox in a three pane vertical view, with the Navigation pane on the left, the Content pane second and the •        Reading pane on the right. This view option is enabled from the View link on the toolbar and selecting Reading Pane On The Right. (Bug 14863)
•        Logos/images can now be embedded with the HTML signature in ZWC. (Bug 25797)
•        Keyboard shortcut list can be printed from the Preferences>Shortcuts folder. Clicking Ctrl+q displays the Zimbra keyboard Shortcuts list. The Shortcut tabs to create your own shortcuts have been removed. Now you can enter a shortcut directly on a page and a dialog displays the options. For instance, type “T” on the Content pane opens a Pick a tab dialog where you can either pick the tag or create a new tag. (Bug 12194)
In the Forward a copy to Mail Preference field, users can specify more than one address to use. They enter a comma to separate the email addresses. The admin can set a limit to the number of forwarding addresses allowed (Bug 11299)
•        Auto-complete is available from shared address books. (Bug 11141)
•        The layout and fields in the Address Book contact form has been improved and the tabs have been removed. (Bug 38907)

Standard Client major enhancements
•        Reading Pane (Bug 31759)
•        Calendar has fish eye view (Bug 33499)
•        Briefcase and Documents support (31123, 31473)
•        HTML signatures are now supported in the standard ZWC. (Bug 28314)
•        The GAL can now be searched from the Standard Client. (Bug 27615)
•        When creating a meeting, the resources are now displayed on the Add Attendees page. (Bug 37952)

Various Task enhancements
•        Up and down arrows can be used while navigating to/from new task boxes and task lists (Bug 19080)
•        The entire task list can be printed (Bug 16456)
•        Users can select multiple tasks and print them. (Bug 31262)
•        Users can print a task that is open for edit. Users can also print an appointment in Calendar that is open for edit and a contact in Address book open for edit. (Bug 25904)
•        You can now select to view tasks by status (Not started, Completed, In progress, Waiting on someone else, and Deferred) Bug 16005
•        You can sync tasks from an iCal remote service. When you create a new task, check Synchronize tasks from the remote service and add the URL. (Bug 17907)
•        Improvements were made to printed messages, address book and calendars to make them easier to read. (Bugs 32996)
•        When printing messages, a Font size link displays on the page to be printed. You can increase or decrease the size of the text before you print. (Bug 17228)
•        Zimbra Mobile. Tasks now sync for ZimbraMobile (Bug 11685)
•        Zimbra Mobile. Mobile security policies that enforce security rules on compliant mobile devices that sync with ZCS accounts can be defined. (Bug 30146)
•        Users can initiate a remote wipe from their Preferences>Mobile Device folder to erase all data from their mobile device. (Bug 18424)
•        Password Policy rules can be pushed to mobile devices and device can be automatically lock when passwords are incorrectly entered. (Bug 35653)
•        A new Preferences>Mobile Devices folder lets users mange their mobile devices. Mobile devices synced to ZWC are listed and users can suspend Sync, resume Sync, wipe the device and cancel the wipe from this page. (Bug 35923)
•        Zimbra Mobile. GAL search was added to the Zimbra Mobile client. (Bug 16532)
•        ZCO. Users can configure to download only the message header information. This makes for faster syncing with the ZCS server and saves storage space on the user’s computer. To read a message, users right click the message header and select Mark to Download Message. Messages marked for download are downloaded on the next sync. Users can also click Send/Receive to download the message. (Bug 11962)
•        ZCO. Personas are synced to Outlook accounts. In Outlook users can switch between personas. (Bug 27302)
•        ZCO. Three issues have been fixed that enable message edits to be reflected on the server:
•        You can now edit the subject and body of messages in ZCO and those changes are reflected on the server (7164)
•        You can now delete attachments in ZCO and that will be reflected on the server (10068)
•        You can now change the To or Cc of a draft message in ZCO and that will be reflected on the server (34680)
•        ZCO. The administrator can configure the ZCO msi file to change the password usage default to require users to enter their passwords before retrieving mail or log in. (Bug 13959)
•        ZCO. Users are notified of new accounts that are shared with them via a Shared link button at the bottom the Outlook screen. They can mount and unmount the shared folders from this link. (Bug 30465)

From the Zimbra forums:

Soon….