Originally Posted by Velo Dog

Anything that will fit will fit...seriously, any tire that will fit between the fork blades or chainstays and under the brakes will work fine. You can certainly go down to 1.4 or 1.25 for road tires, which many people do. If you're riding offroad in rocky or sandy conditions, a 2.2 or 2.3 knobby might fit.
There's a chart floating around that shows theoretical minimum and maximum tire sizes for a given rim width, but I've found it to be very conservative. My old Bridgestone mountain bike is a commuter now, with fenders and 1.25-inch road tires, but when it was my main MB it wore 2.2s on the same rims. My Atlantis road bike has had tires from 25mm to 38mm with no problems.
Don't be afraid to experiment. Tires can make an enormous difference in your bike's feel and performance. You may want to use skinnier tubes with very narrow road tires, but I use 1.5 tubes with pretty much everything. Works fine.