SQL 2005 Build 3186 has a major negative side effect on x64 clusters. Installing it pretty much kills the SQL Agent.. The workaround is to enable unconstrained delegation for the machine and the service accounts. Not exactly a best practices security setting, but necessary. For now, if you are on an x64 cluster and don't have a compelling reason to install this hotfix, you may want to wait until this issue gets resolved.
Here is the MSDN thread discussing the problem and the workaround:
UPDATE: Build 3200 (Cumulative Update 4) has the same issue. Evidently this bug is due to a complex interaction between SQL and Windows Security. I would not expect a quick fix. I can confim that this definitely has major visibility and resource assignment within Microsoft. It just isn't an easy problem to solve.