[BiPolar_Nut]

I've had motherboards go bad (like Mister W mentioned...bad capacitors that domed their tops) that behaved this way and sometimes unplugging the power connection to the motherboard, waiting for the LED on the board to go out, then re-plugging it got it to boot.

It'd be nice if it was the power switch, but if shorting the pins didn't do the same thing as pressing a properly functioning power button, you prolly have a motherboard or power supply problem.

Since it sounds like you can borrow your roomie's power supply, I'd try that and see what happens. Once in my life I had a bad motherboard that would insta-kill power supplies and I fried 3 before I figured that out, but that was an odd problem on much older hardware. There's some risk, but it's very minor. If I was working on it and I didn't see any bad capacitors, swapping a known good PS would be my next step. (or I'd put your PS in a known-good junker box...depending which was easier to get to)