No beeps or anything, if I flip the power switch that is actually on the power supply off and back on, it will fire up the fans (including mobo fans) for about 3 seconds, then turn off. No posting, no beeps, no hard drive whirs.

I pulled the power supply and hooked it up to the mobo on my spare computer and it ran for a minute or so just fine so I don't think it's the power supply.

My next course of action is to reseat the heat sink and see if that helps - my computer has one of those fancy digital temperature monitors but it has never worked right and will often beep to signal overheating after the computer has been on for a few minutes and then 2 minutes later go back to saying the chip is 80 degrees F. (I KNOW I don't have an overheat problem, my house is cold as crap and I have been able to do 3-d gaming for hours on end with no problems. I also have software that monitors temp and I am rarely above 85 degrees, I use my computer mostly for web surfing. However, there could be some internal safety mechanism preventing the booting in error? If reseating doesn't work I will probably just yank the whole thermometer and see if that helps.