I think the mintyboost is awesome, but at walmart i saw a MusicPower Encore for like $30 bucks and thought it could be much cheaper made from scratch. Basically it is just a lithium battery pack that recharges when pluged into ac and has two usb ports that can power devices from the mains when plugged in or from the battery when not.
I think this would make a fantastic instructable; maybe use a cell phone battery(or two) or rechargeable AAs. I can just picture an altoids can hanging from an outlet with a flashing led or something.
Also two usb ports would be a great addition to any mintyboost project.