Our team had fun with wood... arms, chassis, even pulleys and bearing blocks...
http://www.chiefdelphi.com/media/photos/22924
But never tried gears. I'd be tempted to use some nice void-free plywood, like baltic birch, rather than "real" wood, just to eliminate cracking on the grain lines.
The pulley in the photo above looked great, but broke during qualifying at GTR so we replaced it with a plywood one that lasted forever.
Jason
P.S. Ironically, our modified auto mode while we were fixing the pulley found us an invite into the elimination rounds... it was about the only time we did better due to something breaking.