I just solved a problem with some prompting from ladyada, so I thought I'd post:
Isn't it great when site admins participate in the site?
I soldered up my adafruit Motor Shield Kit v1.0, for use with my Arduino Decimila. Plugged it in, nothing. But wait! Remember that the motor shield needs external power. So I soldered on the 'pwr' jumper header that I forgot before, found a 9V wall-wort, and plugged it in. Now the power LED on the motor shield lights up. Great! Twisted a few wires to my Solarbotics Gear Motor GM2, and tried the example DC motor program - forward 1 sec, reverse 1 sec, stop 1 sec. It does something, but it doesn't seem to really 'turn-on', it just jumps or pulses in the correct direction, and then sits. I hook the GM2 to some batteries, and it turns nice and strong, about 0.5 rev/sec. So something is wrong.
I found a forum post that described the problem beautifully, but with no solution, so I posted. Then I thought about the latest Make magazine, 19, describing an arduino robot with gear-motors - these gear-motors often need a capacitor across the leads, so they don't feed noise back into the motor driver and kill it! I used 0.1 microfarad. I also had to add a delay (0.1 sec) between going forward and going in reverse, so the motor would start reliably in reverse. It makes some sense...
Now, on to building a 2 motor/wheel robot, and making it do something!