I just had to upgrade an Exchange 2007 RTM CCR cluster this weekend to Exchange 2007 SP3. This article is still the best reference.
We ran into a problem at Step 5 and Step 14 where the setup program failed reporting that there were still active cluster resources on the node. When we investigated in the Cluster Administrator MMC we found that there was a resource named PBX-ClusterGroup-Servername. After some investigation we found that this resource is related Symantec NetBackup. NetBackup 6.0 contains features that are dependent on a new Common Services Framework (CSF) called, VERITAS Private Branch Exchange (PBX), thus the name.
In any event, if you just take this resource offline on each node prior to the upgrade, you will be able to run it without issue.