My Macbook Pro Unibody (2008) acted quite the same. I did sort of as you explained. Unplugged the fans, powered up, white light burning, no start up tone. Then i forgot about it, to find out that after an hour or so the thing was very HOT! Then put in the cooler for an hour..
Then i tried again. It gave an awfull horn-sound. So I took one Memory bar out. same. The other one out. Same hornsound. both out. Silent! Put one bar back. It booted up! Turn it of again. Two bars in. It booted again! Had Disk Utility put the permisions straight. Turned it of. And turn it on again. It is now booting okay for two days. Sleep is no problem either.
Great! HahaH! Thanks, I thought it was a goner!
Paul