We have updated our SBS 2011 Setup Guide to reflect the need to apply the latest Windows Server 2008 R2 service pack during the setting up of the server.
The Official SBS Blog on the matter:
We have been running Win2K8 R2 SP1 on SBS v7/2011 since the SBS v7 bits were running as our production server here. We then migrated that SBS v7 SP1 server to our current SBS 2011 RTM bits setup.
To date we have not seen any issues with the service pack on SBS 2011.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book