well, i think if i had lots of time, money, and someone who was slightly more mechanical than me, i would be able to make something similar to the gear shifter on my bike. all it is, is a small wheel that turns so much when you push one button, and that tightens the cable, and the other button lets the wheel slip so much, and the springs on the derailleur take up the slack. it's not a terribly difficult concept, and hey, if i think that i can do it, it must be possible. i mean, i'm a programmer and i understand this! :D

also, michael, as i was riding my bike today, i looked down at the front shifter (i have a 21 speed bike), and realized that it's just a piece of metal that touches the chain, only to shove it off to one side or the other. now, that's all i saw, and i didn't have time to examine any more, but maybe we could do something like that, because that would have no problem spinning backwards (and putting in a safegaurd in the software to eliminate shifting while in reverse is super easy). once again, it's just an idea, but one that could work, i think. ;)

Indexing with a motor...
 

I remember seeing something that could aid your mechanical indexer on the cover of small parts. It's essentially a gear shaped like a curved-out star, and each of the points has a track cut into it almost all the way to the shaft going through the gear. It has a matching gear that is comprised of a big cylinder (same radius as the curves cut into the star) and a peg on a metal backing.

Essentially, when the cylinder-looking gear turns, it will slide between two points of the star. The peg will finally come around and get caught in the track of one of the points, moving the star shaped gear a set amount. Then the cylinder will continue spinning, this time sliding in the curved space between the point the peg just moved and the next point on the star.

I'll draw up a picture in paint later, but it's an effective way to apply just a small, precise amount of rotation that will never change with a motor. The only thing you would worry about would be the timing of it.

Geneva Mechanism
 

Oh, Gui! You're killing me :) That was my big secret (TM)

...except that it's not a big secret anymore because I had to ask Andy Baker what it was called.

It's a Geneva Mechanism, and that's exactly what I plan to use :)

Most of the discussion has centered around problems so here is one I didn't see brought up yet. The bike system requires that the bike is moving first and that the chain is not under a significant load at the time of gear change. In all of this, it takes at least 180 degreees of sprocket rotation (minimum) before the new speed is engaged. Of course you can change gears in reverse IF you have two derailluers, but the logic (mechanical) would add significantly to weight. Ultimately, the weight of the system would be the biggest factor here. There is a definite relationship between the length of chain and the amount of reduction/speed changes involved. A 10 speed bike has to allow enough chain length to move laterally about 1.5 inches or the width of five chains. This easy to do over a twenty inch span but not over a ten inch span.
Indexing is easy to do with stepped cams and/or adjustment screws.