Some of the new features: conversation thread, free/busy lookup, sync SMS messages to Exchange, and enhanced voice mail when you have a Windows Mobile 6.1 connected to a Exchange Server 2010 (you will be informed about this update). This update is for 6.1 not for 6.5.
Now you can keep valuable information at hand with the new Microsoft Office Outlook Mobile Update and Microsoft Exchange Server 2010.
The Outlook Mobile Update is available for Windows Mobile 6.1 phones. When your phone is connected to Exchange Server 2010, you are automatically informed if there is an update. If you have a Windows phone running Windows Mobile 6.5, you do not need this update.
Read more at source: http://msexchangeteam.com/archive/2010/03/17/454307.aspx