I know this is now pretty old topic but I came about to fix my Arduino which I fried and want to help anyone viewing this in future. I programmed a new ATMega328 with Arduino Uno bootloader and replaced the damaged AVR chip. Then I proceeded with DFU programming. If anyone is facing the same problem, here are the links:
https://www.virtuabotix.com/how-to-cure-sudden-uno-death-syndrome/
I had a R2 board, so there were no pins in the back to solder. So the instructions don't say what to do in that case. I found this topic about the same problem:
http://forum.arduino.cc/index.php/topic,61100.0.html
Alternate way to enter DFU mode:
http://forums.adafruit.com/viewtopic.php?f=25&t=19751
That's all. Very happy to revive the dead Arduino. Thanks johnwasser!