MacBook will not turn on
There is a problem with the NVIDIA graphics processors that can prevent the MacBook Pro for powering on. I have a 2008 15" MacBook Pro that had symptoms similar to what you are describing about a month ago. I took it to the Genius Bar and they determined that it was the NVIDIA chip causing it and replaced the logic board for free despite the fact that it was not under warranty and I didn't have AppleCare.

My recommendation is to take it in to the Genius Bar and have them check it out. I have found they have great support and have fixed problems out of warranty (such as this one) on more than one occasion. However, it looks like from the comments on another answer that you have done this already. It might be worth a shot trying again and take the support article with you to see if they change the diagnosis.

Or perhaps this answer might help someone else experiencing the same problem.