I'm using an arduino mega. I have had the board for a while without any problems. My current project is to convert a 27 Mhz rc car into a wifi controlled car that accepts inputs from my computer. I got as far as a rough prototype where the mega could use the asynclabs wishield. It takes a lot of effort and manipulation to get it all working, but i had it working very well. I was trying to change how i was powering the mega board.
I had a 7.2V nicd battery connected to a 2.1 mm power connector and then plugged it into the arduino. That worked fine and i was able to verify that it was all working correctly. Then i decided to try and use the same 7.2V battery for both the car supply and the arduino. I stopped using the 2.1 mm adapter tip and used the Vin whole instead. I'm not sure what went wrong, but the board started getting extremely hot and smoking before i could unplug it. After that I can't get the board to connect to my computer. The board doesn't even show up as being connected to my computer. (I have an apple macbook pro and i've tried both windows 7, and mac os x 10.6.2 neither one can detect the board.)
I've tried connecting the arduino to the battery using the 2.1 mm tip again to see if i just burned out the FTDI chip, but the program doesn't run anymore. The board seems to be pretty inoperative. When i have the board and the board alone connected through usb it will turn on the power light and the green led, but it isn't detected. When i connect the 7.2V battery through the 2.1 mm tip the voltage regulator next to the DC jack starts warming up, and the FTDI chip gets pretty warm too. I'm not sure what i fried, and if there is an easy way to replace the damaged piece.