Replaced Mobo; PC Turns On By Itself Now?

The motherboard in my PC recently failed, so I RMA'd it to ASUS. The replacement board seems to power the computer back on a couple of seconds after I shut it down. The only way to keep it off any longer is to flip the switch on my PSU. This happened when I was breadboarding it at my office and when I had it fully assembled in it's case at home.

I figure there is either a short somewhere, or a jumper setting that is causing the problem. Does anyone have any idea why it would be doing this? Everything else is fine, it never reboots without warning or behaves strange; it just turns back on after I power it off.

System Specs:
i7 920
ASUS P6T Deluxe
Corsair 850TX
Corsair XMS3 DDR3 1333 (3x2GB)
EVGA GTX 275
Creative X-Fi Titanium Fatal1ty
2x WD Caviar Black 500GB (RAID 0)
1x WD Caviar Blue 500GB
Windows Vista Ultimate x64
Antec P182

The reason I find a case short so hard to believe is because it was doing the same thing when it wasn't even in the case while I was breadboarding the system to test it's functionality. At the time I thought it had something weird to do with the old surge protector I had it plugged into, but it did the same thing at home.

Also every time I've experienced a case short, the computer refuses to even power on and otherwise behaves strangely; this just turns itself back on.

I browsed through the 200 page .pdf user's manual, but found nothing helpful. I'll probably restore my BIOS to default settings and/or flash it to update it and see if either has any effect. I did notice when I first got the board that it had collected some dust on the I/O ports and someone wrote a letter in sharpie on one of the smaller chips on the board. I assumed this meant the board had been tested and was probably a refurbished unit.

I'd hate to have to send another board back to ASUS and pay for shipping again, so I'll probably just learn to deal with it unless it causes and problems beyond this little annoyance.