Well, I want the portability. If I'm within reach of a wall plug, I may as well pull out a laptop If an undervoltage won't be very dangerous I'll just go with the most readily available parts; which in this case would probably be the voltage regulator.

Has anyone else built one? Any cool additions? (I know Simon had a blinking light when power got too low, both he and Thoughtfix added a switch (which I'll do as well given the simplicity)).