Is there a way to patch a clustered SQL Server 2005 instance running on two physical hosts without any downtime (or just the downtime during a cluster failover)? 

 I'm asking this because my DBA claims the clustered instance cannot be available (on either host) during SQL Server patching, which will take about 20-30 minutes.  However, I thought that was the whole point of clustering -- one of the cluster instances will be active while the other is down for some reason (hardware, network, and even patching?). 

 Any guidance is much appreciated.

Your DBA is right, there is no way you can apply service pack without downtime.

Make sure you apply the SP on both nodes.

High availability only incase of server hardware failure not the SAN failure and software upgrades

Thanks for the quick response.  We will think about setting up database mirroring between this 2-node clustered instance and another instance. 

This might be a stupid question but can we mirror databases to another clustered instance running on the same 2 nodes?  I realize that might lead to a situation where both the mirror and the principal instances are running on the same node.  Not sure if that's possible...