12v into 5v input damage

G'day
I have an arduino duemilanove and I accidentally left my power supply on 12v when I pluged the 5v pin of the Arduino into it and left it on for a few seconds. The ATmega328 was warm after that.
It still shows up on the IDE and the power LED lights up when I plug it in.
When I try to upload a sketch the red RX LED flashes a few times, then the IDE comes up with an upload error.

Is the problem easy to fix?
Will I have to replace the ATmega328 or is there some sort of fuse somewhere that I can reset, or could there be other parts damaged on the board.

Thanks
~Josh

Is the problem easy to fix?
Will I have to replace the ATmega328 or is there some sort of fuse somewhere that I can reset, or could there be other parts damaged on the board.

Search for and run the 'loop back test'. If the test is successful it means the FTDI USB serial converter chip is OK and you would probably only require a replacement 328p chip with bootloader installed. Typically around $6.

If the loop back test fails then your probably better off just getting a new board in my opinion.

Lefty

Loop-back test...
http://arduino.cc/forum/index.php/topic,73748.0.html

Great. The board passed the loop-back test, so I will reuse the board. I have ordered some more ATmega328s from SparkFun to replace the uC.

Thanks!

Be aware that it might still be damaged (ie it appears to work but not everything works or works reliably). 12V into a 6V max chip is a severe overload - don't be surprised if it gives up at some random point in the future.