Grey Screen Boot, Open Firmware

Verify that the Airport Extreme card is firmly seated. I've seen cases on the 12" PowerBook where if it's just slightly out of its socket, the machine will go into open firmware, or otherwise act strange.

Beyond that, is it a faint, sort of opaque grey screen, or a solid white screen? The odd-colored grey screen can be an indication that the inverter light is working, but the video signal is not coming across, in which case I'd make sure the video cable is firmly seated on the logic board side. I'd try booting in option mode, i.e. power on with the option key held down, and if you DON'T see the option menu, that generally means you have a video-related issue. You can also try the external monitor as Mayer suggests.