If you have problems synchronising you’re Windows Mobile 6.5 on Windows 2008 RC2 and Microsoft Outlook 2010 with the message “Outlook is not the default email client”, you can set Microsoft Outlook 2010 as default email client in Microsoft Outlook 2010:
File > Options:
Check “Make Outlook the default program for E-Mail, Contacts, and Calendar and ”Click on “Default Programs”
Check the Select all checkbox.
Install Windows Mobile device centre 6.1 (Microsoft Windows Mobile Device Center 6.1 Driver for Windows Vista (64-bit).exe). Sync you’re Windows Mobile 6.5 and uncheck the One Notes sync
This worked for me, I am know syncing my HD2 Windows Mobile 6.5 phone with Microsoft Outlook 2010 on a Windows 2008 RC2 x64 server.