Just saw this thread, I've encountered this problem on my staging server (not clustered) and today on my production server (clustered). For the record, both servers are 2008 R2. It seems that this happens after I restart the services (or take offline/bring online on the cluster), the SQL Server Agent service appears to be online, says it's online and if I close and re-open the Job Activity Monitor it opens fine and shows my jobs are enabled and ready to run, but the time passes and the jobs don't start.

I'm interested if anyone with this issue found a solution. Restarting the Agent Service usually does get things rolling.