The new service pack consists of two executable files which run in sequence: WSSv3 SP1 and then MOSS SP1.
If the Service Pack doesn’t work, the upgrade will claim that it was complete, however, the upgrade may have actually crashed halfway through the WSSv3 SP1 portion of the operation, leaving half of the databases updated and the other half (the content and SSP databases) not updated. The result is the content databases are locked, and the SharePoint services were hung up in a loop. In short – If this occurs, you have a problem. Feel free to email me for details.
Resolution: To fix everything, run a series of stsadm commands to force the upgrade, because the GUI-based commands will not respond and the service pack will claim that it was already installed. Then get all of the timer jobs to start running again. This should do it.