connecting the motoshield makes arduino lose power

My gf and I are trying to get the motoshield working. Yesterday, we used headers to plug the motor driver we got from sparkfun into our arduino uno. It didn't really work. It did at times if we twisted the shield a bit, but we realized that we needed to solder the headers onto the motor shield. We did that today. My gf was soldering and accidentally soldered some of the pins together, but then she removed it with the iron. When we hooked it up to the arduino, nothing happened. Once we removed the motor shield, the arduino powered on and worked normally. When we plug in the motor shield again, the green power LED on the arduino goes out and turns off.

Any idea what's going on, or how we can find out what's going on?

Sounds like there is still a solder bridge somewhere on the shield board causing the board's 5 volt buss to be shorted out. More inspection is needed or possibly troubleshooting with an ohm meter looking for short circuits.