In alt.comp.hardware.pc-homebuilt "Al Franz"
Depends on what you're screwing into and the thickness of the drywall.
For 2x4s and 3/8" drywall, I'd recommend about inch and a half screws.
For sheetmetal studs or thinner drywall, use shorter screws.
But it's mainly a matter of taste or familiarity.
http://www.hometime.com/Howto/projects/drywall/drwl_1.htm