SSIS subsystem failed to load
Customer's weekly maintenance plan failed to run:
...Maintenance Plan,Subplan,,Unable to start execution of step 1 (reason: The SSIS subsystem failed to load [see the SQLAGENT.OUT file for details]; The job has been suspended). The step failed
SQL 2005, RTM, 2 node single instance cluster on Windows 2003
Lots of checking later, the problem turned out to be very simple:
Install SSIS on the node it was failing on.