My PSU brand is OCZ. Here's what I've done: First, I went ahead and dismantled the entire thing and even extracted the motherboard. I did not notice any extra motherboard mounting screws underneath, so that's fine. I then placed my motherboard on a piece of cardboard, plugged in the PSU and fired it up...only it did the same thing, no power up!
At this point with the motherboard completely separated from the case, there aren't any intermittant connection issues with the case itself or some other unkown object. So, like you've all said, it must be either the PSU or motherboard. However, might the connected CPU still potentionally play a role in this thing? I don't know.
I guess I'll go back and test every rail coming off the PSU with my friend's multi-tester. Here's one thing...when I turn on the PSU while it's connected to the motherboard, two lights light up on the board and stay on even while the motherboard won't power up...so the board is getting power. Would this lead us more toward the motherboard being the issue? Of course, at the same time, the motherboard obviously is able to except this power trickle and light up those lights, so it looks perhaps like a draw...sigh.