This is inspired by a wooden toy I saw at my niece's house. The middle of the worm rises and falls as you roll the worm along its wheels.
The inchworm is designed to print without support material. The articulated body prints assembled in one piece. The wheels snap on - no hardware or glue needed.
August 8, 2013 update
Version 2 of the inchworm is now available. This version uses an axle to allow the wheels to roll together. The wheels now have some tread. Try adding some rubber bands around the wheels for more traction.
For version 2: print the inchworm body, one axle, and two wheels. The body prints assembled in one piece. The wheels and axle should snap together with a snug fit.