Q: In SQL Server 2008, can you apply a service pack to the passive node in a cluster?

As it turns out, yes:

Apply a service pack or hotfix SQL Server 2008 to a failover cluster instance - Microsoft SQL Server - Site Home - TechNet Blogs

In SQL Server 2005, service packs had to be installed on the active node in a cluster. SQL 2008 removes that constraint, thus reducing total downtime.