AC Power?

What would happen if I were to connect an Arduino to a 9V 500mA AC adaptor instead of DC? Would it just not work? Would it explode in a huge ball of flames? What would happen?

If you connect it to Vin then there is a series diode fore reverse voltage protection that would act as a half wave rectifier on most arduino boards. This would turn it into DC but with quite a bit of ripple. I would be tempted to add an extra large electrolytic capacitor on the cathode end of that diode. On the Diecimila there is a 100uF in this place. I would be tempted to add another 1000uF or so. Remember 9V RMS is about 13V peak so there is a dissipation issue if you draw much current.

In my case, when I did that, it let the magic blue smoke out of a caps for the on-board voltage regulator. I didn’t pay close enough attention to the wall wart, it said 12v AC, not DC.

-Chase

it said 12v AC, not DC.

12v AC gives you 17 volts peak. So you are feeding it with 17v.

I suspect that your supply gave you even more than 12v due to the light load and exceeded the voltage rating of the input capacitor.