Namely, for 2008 R2 support, you need a Server 2008 R2 specific hotfix (KB 975858) and modify a few NTLM security settings that differ as defaults on Server 2008 R2 compared to Server 2008 R1.
Ideally, if you can support it in your environment, I would configure your servers and clients to be the most secure by default with use of a GPO or registry setting. That way, you are using the latest NTLM protocol with 128-bit encryption.
In the screenshot below, it shows the security setup on my laptop for client security:
Keep in mind, it is best to match these settings on the client side and server side.
If you configure the client side and do not configure the server side, you will still be able to connect.
If you configure the server side, and leave the client side at defaults, depending on the OS release, connection problems may happen due to the mismatch.
You can also read the official Microsoft guidance on Server 2008 R2 and OCS 2007 R2 here and here.
Update: I've discovered some additional issues that can be worked around. I have documented them in a new entry here.
The big news with this release is support for Windows 2008 R2 and workarounds for specific VOIP inter-op scenarios. I will be publishing some more info on both of these items later but wanted to get the word out because this update has been a long time coming!