The back tire wears out much faster than the front. When I get a new tire, I move the old front to the back, and put the new one on the front.
Sheldon's site says - don't rotate tires to even out the wear, but it's OK to move the front to the back.
Sheldon on tire wear:
if you are mainly concerned with safety/function, there are only two reasons for replacing old tires:
1. When the tread is worn so thin that you start getting a lot of flats from small pieces of glass and the like, or the fabric shows through the rubber.
2. When the tire's fabric has been damaged, so that the tire has a lumpy, irregular appearance somewhere, or the tube bulges through the tire.
Cracks in the tread are harmless. Small punctures in the tire such as are typically caused by nails, tacks, thorns or glas slivers are also harmless to the tire, since the tire doesn't need to be air-tight.