Issue #1: Using the OCS 2007 R2 Administration tools on Server 2008 R2 has shown to be problematic for moving users from LCS and OCS 2007 R1/R2 to other OCS servers. For now, it is best to use a non-Server 2008 R2 workstation or server to move users to and from OCS servers. You can 'force' the users to move, but it doesn't always result in a good move. So, avoid it if possible.
Issue #2: If you are trying to run "ABServer.exe -SyncNow" on an OCS 2007 R2 server running on Server 2008 R2, you will need to create a new DWORD registry entry at HKLM\SYSTEM\CurrentControlSet\Control\Lsa\DisableLoopbackCheck set to the value of 1.
Due to .NET 3.5 SP1 on Server 2008 R2, if this registry value is not set, you will receive an error message of "Triggering Address Book Server synchronization pass - function not available when RTCSRV service is not running. Start the RTCSRV service." A reboot may be required for the new registry setting to take effect.
Usually, manual regeneration of the address book is not required since the front end will refresh the contents every night by default. It is only the manual run of ABServer.exe that is affected on Server 2008 R2. It is normally best to leave DisableLoopbackCheck disabled (set to 0) if possible for security reasons.
Thankfully, OCS seems to perform very well on Server 2008 R2 once the known issues are worked around.
Microsoft keeps an updated KB article about these issues, which do not have these documented yet, here.
I also have my original guidance for OCS 2007 R2 on Server 2008 R2 here. I will work on getting these incorporated into the original post when I have a free moment.