I have been using Sketchup for a few years now, and the other day I made a mock up of a part on our robot in <30 seconds it seems like. While machine shops won't accept the files (the machines that take computer files won't take it(as far I know)) it is a great way to get ideas in 3d, as you know. A few other kids on 1793 have tried it, and have gotten alright at it, to where you can tell whats going on, and thats what counts.
Another thing to look into is
sketchyphysics, a plugin with a basic physics engine so you can get some basic driving or moving whatevers.
Here's what I've done:
http://sketchup.google.com/3dwarehou...3567&scoring=m
P.S. look at my signature (fourth one on the left)