Smoking hot motherboard! Litterally... :(

So I just built a new system (I've built many in the past as well) and it worked great for half a day or so. I left it running before leaving for work and when I got home that evening, it was off. I tried turning it on, but the fan lights and fans sputtered very briefly (perhaps a tenth of a second), and then nothing. I did a fair amount of testing and eventually discovered that if the motherboard's CPU power cable was left unplugged, the machine would not immediately shut back down (but obviously wouldn't do anything with no CPU power). At this point, I suspected there might be a short somewhere that was causing the machine (or at least power supply) to cut out. I took the motherboard back out of the case and had only the motherboard, CPU, RAM, and power supply hooked up. Tested it out, same thing. I tried the CPU in another machine, and it seemed to work. Finally, I tried a different power supply. This power supply did not cut out immediately like the other one. Instead, the suspect CPU power connection on the motherboard started smoking horribly, at which point I cut the power on the PSU. At this point, I feel pretty certain that this must have been a short in the motherboard or perhaps the CPU power connector on the motherboard. However, I'm left wondering two things:

1. Is it possible that the original power supply itself was the root cause of this problem? Seems unlikely, but I'm really not sure...
2. Is it possible that when I used the second power supply and caused the motherboard/CPU power connector to start smoking that the other connected components (CPU and RAM) were damaged?

I'm trying to decide whether to simply replace the motherboard, or the motherboard and the PSU/CPU/RAM. I have a time limit to RMA these parts! Thoughts?

Thanks!
Ryan