Random thought: Has anyone ever made something like the equivalent of the Serotta Fit Bike (adjustable top tube, angles, stem length, etc) but actually rideable? That would be an awesome way to experiment with geometry on the go as opposed to on a stationary platform with no wheels or anything.

I'm thinking instead of welding the tubing together into a fixed geometry, you could just thread the ends of each tube and have it screw together into threaded lugs. You would have a set of threaded lugs with slight angle variations which would allow you to play with seat and head tube angles. Then you just screw the whole frame together and tighten it down with washers (like on a threaded headset). This would let you fine tune tube lengths and angles, and the construction might be rigid enough to be rideable (gently) so you could try the geometry out. Seems like the perfect tool for custom frame designers.