I have not upgraded a Cluster to SQL2008R2 yet. I should upgrade a SQL 2005 to SQL 2008R2 in the next two months and my plan is to:
1.Full system backup backup of the passive node
2.Upgrade SQL in the passive node
3.Take a last Full backup of databases in their 2005 version
4.Failover from SQL 2005 (active) node to upgraded SQL2008R2 node
5.DBA Check / User Acceptance Checks
6.Upgrade the now passive SQL2005 Node
7.Take a full backup of the databases
8.Failover to the newly upgraded node
9.DBA Check / User Acceptance Checks