Microsoft had scheduled support for OCS on Windows 2008R2 in 1Q2010. On the last day of the quarter we finally have confirmation of support.
There are a few still a few issues, Group Chat support being the main one.
Support is also only for new installations of Windows 2008 R2 you cannot upgrade an existing OCS 2007R2 system.
Two knowledge base articles have been released
When you upgrade a domain controller to Windows 2008 R2 x64 OCS does not function correctly. To fix this you need to rerun the ForestPrep. Full details are in this knowledge base article 982020
For full details and the process required to install and prepare Windows 2008R2 for OCS look at the knowledge base article 982021