I made an article on this, go here: Arduino Playground - WallOutlet
What's with the spam in the article there?
You should update the page with some links to where to buy the little USB power adaptors. For example, I found this one at ThinkGeek.com
http://www.thinkgeek.com/gadgets/travelpower/9124/
Or the one that comes with (or used to) an iPod: (kind of expensive)
I use my iPod adapter to power mine. BlackBerry phones also have a USB power adapter.
huh, idk where the music power duet is made, so i cant help
but good point