After searching for an hour or so for a wall wart that was the right size and polarity, I remembered that I had a a Radio Shack wall wart that had different sized barrel plugs, now what if the Arduino had interchangeable barrel jacks.
Well, you do have access to Vin/Gnd on the Power header, all you need is two wires to connect to whatever jack you want to use...
I know but if you do not want to take apart a good wall wart.
You can get different sized plugs online that will make with your wallwart. Just need to look around more.
I'd look now, but I'm walking out the door to go home.
5.5/2.1 is the arduino size.
5.5/2.5 is another common size.
Look around, you can find them.
I know, this was just an idea.
I know but if you do not want to take apart a good wall wart.
but using a bad one makes no sense 
Check second hand stores, they often hav a big box with different wallwarts for cheap.
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, 1000mA
Switching DC Wall Power Adapter 7.5V / 1A - dipmicro electronics 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.
barrel jack
Power Jack 2mm 24V / 3.5A w/ Built-in Switch - dipmicro electronics
I personally just run off 5V wallwarts, and don't even use regulators.
Such as
Switching DC Wall Power Adapter 5V / 1A - dipmicro electronics or
http://www.mpja.com/prodinfo.asp?number=18520+PS
with a panel mount connector on the enclosure wall
http://www.mpja.com/prodinfo.asp?number=18549+PL
CrossRoads:
I personally just run off 5V wallwarts, and don't even use regulators.
But that output is not always clean or exact, I have noticed as much as +6 volts (greater than the output) ripple on wall warts. and a 9 volt reg being 11.5 volts
The 5V switching regulator wallwarts don't do that. They put out rock solid 5v.