UPDATE (2/11/2011): The Exchange Team has released a statement that Windows 2008 R2 SP1 will be supported with the following Exchange versions:
- Exchange 2007 SP3
- Exchange 2010 RTM
- Exchange 2010 SP1
As of yesterday, Microsoft announced the release of SP1 for Windows 2008 R2 and Windows 7 (see here). Currently the Exchange team has not announced support for SP1. The Exchange team will soon release a statement and update the Supportability Matrix that will show official support. Most likely SP1 will not be supported until SP2 for Exchange 2010 (but again, nothing is official yet).
I will post again once the announcement is made, but the official word will come from the following places:
The guidance is to always wait for official support from Microsoft before applying any updates to your Exchange Servers.