I was thinking about cutting a usb cable, put 5V and Ground on an end with a regulated voltage source, and connect the other end to the Arduino USB input. Is that a bad idea? What should I do with the data cables? Connect them to the ground? With a resistor?
That should work fine for powering the board. It really doesn't matter what you do with the data signal pins, I would just cut them and let them float. Keep in mind that the maximum +5vdc current avalible to the board will still be limited by the on board USB thermal fuse rated at 500ma.