So I recently saw a blog post on hypocycloid gears on the Reprap Blog ( http://fabricationsofthemind.com/2010/07/09/extruder-design-1-printable-1001-hypocycloidal-gearbox/ ) .
Some really good info on them can be found here:
Also do a search on youtube for hypocycloid reducer. Lots of vids.
They seem to be perfect for a robot arm joint. I want to make a robot arm. I started to put one together in OpenSCAD. I have way more than I'm uploading here, but this is just the arm joint. The outside layers are fixed and the inside four layers rotate as the gears turn. I wanted to cut this using ponoko bamboo, but there is no way to go from OpenSCAD to ponoko bamboo (that I know of). I don't have a makerbot yet.
Hopefully someone can pick this up and run with it. The file needs to be edited quite a bit to print it out on a Reprap or something. Drop me a message if you want to print this out and I can help you understand the file.