If it turns out not to be the PSU (which - as mentioned above IS the most likely thing) or a mobo-case short, I have another couple of suggestions.
1. A lot of the recent Asus mobo's have passive heatsink's on the northbridge which have a layer of the disgusting bubble-gum type thermal goop underneath, which in many cases is very poorly applied. On several boards that I have seen there has been a big "bubble" of air in the middle stopping the heatsink from doing its job. Try removing the heatsink (its not too difficult), removing the compound there, and replacing it with some real stuff (A.S. 5 or Ceramique - ceramique is my preference because its easy to apply).
2. Check (just in case) that there is no evidence of "leaky" capacitors on the mobo