I'd go back to an Apple store, hopefully a different one, and try again. Granted I don't have the terms of the Apple warranty memorized, but it seems odd and not quite right that they would not honor the warranty because of a bump on the screen, especially since the issue is that it does not power on at all. I don't know of a way that subtle screen damage could kill the board.
So, yeah, I'd go back, find another "genius", be really nice to the person, and potentially even buy extended Applecare for the machine, or perhaps an iPad...a gesture such as this might give them incentive to help you. I would even go so far as to mention to them you're willing to buy Applecare if they help you out. They do have a lot of discretion as far as how they end up dealing with a situation, so in turn, the way you act and present it to them also makes a big difference.
On another note, if you haven't already, I'd try reseating the RAM and verifying that the AC adapter is good, as well as performing the various resets on the machine.