Just as an alternative here is a manual power on, auto power off circuit I've build and used in a few projects. It's based on a small DIP packaged 5vdc single coil latching relay that I found on Ebay ( I think I paid $5 for 10 of them). It draws no current when it's powered off, just brief coil current consumed while switching. On power up the output pin is set low in the startup function which 'seals' the power on contacts on the switch. Once powered up the sketch can turn off the system by simply outputting a digital low on the relay coil pin.
http://img25.imageshack.us/img25/563/08miq7.jpg
Lefty