Hahaha, at first I thought this was another "the MP line is discontinued thread". Happy to see it isn't !
Now to answer your question :
If absolutely nothing happens when you press the power button, yes I could potentially be the power supply (which I pray for you it is, rather that the logic board)
I your type of situation, I doubt forums will really be able to help you as it might be a hardware problem very specific to your machine.
I would try and switch different components with an other mac pro to be able to identify the problem but most people don't have that type of hardware handy.
My advice would be to take it in at your local Apple Store and get a quote...
Good luck !
edit : this is my post #100 ! Hope it helps