Remove completely the top case (unplug it) and try to boot the machine directly from the logic board. Search for the power icon (same as the one on your power button) on the logic board, near the icon there should be two small solder pads. Just briefly bridge the two pads with a small jeweler flat screwdriver (ac adapter must be connected). If the machine power on then your top case power button could be faulty, if not then the DC in board (or the logic board) could be the problem.
Here's a pict from a macbook logic board. Search for the same power icon on yours or PWR letters.
[image|108281|size=thumbnail]