I have the same problem on my A1150 Mac Book Pro with a grey screen at boot up after the apple logo, even after a SMC reset, and PRAM reset, same issue, but I can boot using a safe boot, although the mouse has a small checkered box once I am in OS X 10.6 Snow Leopard. I ran disk utility in Safe boot from another disk partition with another copy of OS X 10.6, and found no problems with bad blocks / inodes on the hard disk, or disk permissions issue's.
This 2006 Mac Book Pro computer was given to me by an Apple consultant who told me that it would only boot into a black screen so his customer gave it to him when he upgraded him to a new Mac Book pro, it also had no video using an external monitor.
I googled the black screen issue, and found a you tube video on "How to Bake My Mac".
I thought what do I have to lose, a new logic board would cost more then what these sell for on Ebay, so I thought I would see if this was a hoax or would actually work, I also wrote to mythbusters first and posteed on their facebook page asking them if this would work, but after several weeks with out a reply, I decided to use Ifixit to see how to take it apart, and removed the logic board, which has an embedded ATI 1600 GPU. I triple wrapped the logic board in foil, exposing just the GPU chip, and placed it in the oven to re-flow the GPU chip solder, after it cooled down I put the Mac Book Pro back together and installed SMC fan control to raise the speed of the cooling fans in hopes to keep the CPU, and GPU cooler, and it's been working for several months with out issue, until this past week.
Could this be an issue with the PRAM battery causing the grey screen, or possibly the GPU needs to be re-balled with a BGA reworok station for a more permanent repair?
Since it's a 2006 Intel Core Duo, and not a Core2 Duo CPU, it's just not worth buying a new logic board or paying the $150.00 to have it reballed.
Jose F. Medeiros
The Unemployed IT guy!