There are some reports on the newsgroups of storage validation failures after upgrading 3+ node clusters to Windows Server 2008 R2 SP1. It seems Microsoft is aware of the issue and they are investigating. I have been following this thread to keep up to date on the issue

http://social.technet.microsoft.com/Forums/en-US/winserverClustering/thread/e67026af-7f9b-4698-9ca7-11853431ed86

You might want to wait for some sort of resolution before you apply SP1 to your clusters.