The biggest part is software -- getting a sun positioning algorithm to run on arduino, then modifying it to accomodate multiple heliostats pointing at a common target a la Gabriel at cerebral meltdown. But, WOW. I just looked at his site to add the link and he's done a lot since the last time I visited. He's working on an arduino platform now and already has sun tracking working and, he has it working for multiple heliostats and multiple targets.
Next issue is how to cheaply run lots of motors off the same hardware that is doing sun tracking. Gabriel has a solution for this too, here. I've got it a little easier in that my motors only use 400mA at 3V. I can probably use something like a ULN2803, that could power eight of my motors no problem. A board with 4 of those plus the shift registers and some nice connectors for getting wiring out? Probably $5 in parts to control 32 motors = 16 heliostats. All in for a 32 'stat array? Arduino ($30) + time clock ($20 -- cheaper than GPS) + motor drivers (say, $30) = $80 = about $3/stat. As long as it's under $5/stat, it's acceptable.
Anyway, despite some Makerbot delays, seeing other people's work (there are lots working on this) is reinvigorating. More to come.
UPDATE: after a little homework, I found this article, a great review of motor controls. It provides a circuit using a ULN2003A chip and points out that you need to add DPDT relays otherwise you can't move the motor both directions. It also says a very cheap way to do it is with a circuit called a half bridge made out of only a very few, cheap parts. The only reason not to use it is it requires 10mA to signal, but the Arduino provides that and shift registers appear able to pass it through also. It's high efficiency too so is certainly worth trying especially in a high production environment. It's designed for 3V motor that draws 300mA which is very close to the Solarbotics motor's specs except that it draws 400mA at stall.
And, checking back with Solarbotics, they now have this motor, which is very similar to the one I'm using but is even more efficient, producing almost the same torque at 3V but with a stall at 253mA. So the half circuit would work as designed. That pairing is attractive longer term.