Make sure you find one with DC output tho, not AC.
I once bought a Midi sound module via e-bay, the original seller said it turned on, but he couldn't get sound out of it.
Turns it he had a DC wallwart, but this device actually required an AC output module!
For Arduino, make sure to get DC.
Ijust received these for my '1284 boards.
7.5V DC, 1000mAhttp://www.dipmicro.com/store/DCA-07510
plugs right into the Duemilanove jack very nicely!
Figure why make the regulator have to drag 12V all the way down, why not give it a head start and let it run cooler.
I personally just run off 5V wallwarts, and don't even use regulators.
with a panel mount connector on the enclosure wallhttp://www.mpja.com/prodinfo.asp?number=18549+PL