I got the steel Motobecane fantomcross Uno from bikes direct, ~$400, acouple months ago. I upped the freewheel to 20T and combined with the 38T front chainring I have a pretty nice gear ratio for off-road fireroad and trail riding. It comes with a flip-flop hub and the 16T fixed gear is nice when I ride with my road bike friends.

Apparently it can fit 42c wide tires, but I think only in the front. I currently have 38c tires on and the clearance between the back wheel and seat tube is about a milimeter. So far it's a great offroad bike, super snappy and responsive and the only maintainence is in lubing the chain. I think it has fender and rack mounts too so it could be a good commuter.