When I visited Makerbot Industries last month I came away with some of the windup walker mechanisms for a special project I have in mind.
The first step was to create a 'negative' model of the space needed by the walker mechanism. That took a few attempts, but finally I managed to get the dimensions and geometry close enough to be functional.
Then I modeled a shell for the robot. In this case it was extremely simple - just a proof of concept. Once I had the shell modeled, I subtracted the negative model, exported the STL, sliced it using Slic3r 0.8.2 (corrected 8/2/2012), and printed it on my Tantillus RepRap printer.
The print turned out very well and only required some minimal cleanup to remove a few stringers inside the shell. The walker mechanism slipped in perfectly the first time, and the robot was off and walking.
Now I can get creative and design some shells that actually look like a robot.