Just closing out this thread.
My Rosewill 500W PSU crapped out on me the other day so I purchased an OCZ 750W ZT PSU. The new PSU will now let my computer sleep or shut down without turning the fans on full blast. Also, I can reboot my machine and wake it from sleeping without toggling the rocker switch on the back of the PSU.
So, verdict: Faulty PSU.