I think you have to be careful here about failing to a lower SP version. Some SP versions will rev the database level, which means a previous version of SQL can't read the files. You'll have issues.
You should apply an SP to a node, fail over, and then apply to the first (now secondary) node. If you cannot apply patches to a node, then you want to remove the SP patch, but if you've already moved the dbs to this node, SQL doesn't downgrade the database. Most Service Packs don't rev the internal database revision, but some do.
Perry, I'm not sure what you see in that kb, but am I missing something. Doesn't appear to say SQL will downgrade. The admin needs to downgrade before running SQL Server on the node that's been updated. IIRC, SQL Server 2008 SP2 introduced the 15k partition limit, which rev'd the database internal version level, so SP3 shouldn't be able to go to SP1.