Started off trying to test the PSU. Shorting GND and ON seems to power up the PSU. (From here http://pcsupport.about.com/gi/dynam...) Actually there was another connector hooked to the motherboard (not sure of the name) that I didn't take off and when I shorted it the motherboard powered it up. So, that makes me think maybe it is the power switch. I tried the screwdriver trick to short the pins at the motherboard but nothing happens. Suggestions?

You have eliminated the switch, but you haven't eliminated the power supply. The power supply needs the power good (PS-ON) signal from the motherboard to come up completely. You supplied that power good signal by jumpering the power good line to ground. The motherboard will not supply that power good signal unless all the voltages are within tolerance. Of course it's possible that the voltages are ok, but the motherboard power good circuit is failing.

Hmm, I'll start with a new PSU and go from there. Thanks for the reply.

Ok, went out and picked up a new PSU - didn't work. Any idea what readings I should get using a multimeter on the motherboard pins where the power button leads plug into? My reading is something like 2v. (note my multimeter is not digital). Is this even something worth checking? Any other suggestions?

(edited) Forget about checking the voltage on the power switch connections. I hope you got a compatible power supply. Compare the color of the wires. They need to be very similar. It could be a motherboard problem or something connected to the motherboard that is causing the problem. I would disconnect all the devices from the motherboard. Pull all the cards including the RAM, but not the CPU. When you power up, you should get some error beeps (if there is a speaker connected). Getting error beeps is good. That means the motherboard and CPU are working and something connected to the motherboard is probably defective.