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
Intend to upgrade to 3+ node clusters?
You might want to wait for some sort of resolution before you apply SP1 to your clusters.
Reproduced with permission from https://clusteringformeremortals.com/2011/03/31/you-might-want-to-hold-off-on-the-sp1-upgrade-for-your-cluster/