I have had two macs where the power button gave me a problem.
My advice? If the machine is fairly new (one of mine had a problem straight out of the box - the other about three days later where, like yours it wouldn't come out of sleep mode and once unplugged and replugged to the power it wouldn't then start at all) talk to apple and insist on getting a DOA code - this is Dead on Arrival. The machine can be replaced. They don't seem to like doing this but rather prefer to fix the machine. My argument was that I didn't want a fixed machine but one which worked from the go.
They then send a tech person round to certify the problem and then will send you a new machine. It can take a while and they will kick up a fus - but I found that the second time - once I was asking for a DOA code and they realised that I knew what this was and how it worked - the process was a lot simpler.
Good luck
