In a very informative post over at Technet, Mark from Microsoft detailed what features Exchange Server 2010 will bring to Exchange Activesync users on Windows Mobile.
Most interesting is that the Exchange server can actually push out a new version of Outlook over the air to Windows Mobile 6.1, meaning older devices wont be left out of the party.
Other useful features are SMS sync and the ability to send SMS messages from Outlook, threaded e-mail views, streaming and transcription of voice mail, and free and busy status views.
Manageability of mobile devices have also been improved, with the ability to blacklist devices and block whole classes of devices, meaning insecure backdoor devices like the iPhone 3G can be stopped even before they log on to the network.
For the full list of features see Technet here.