The newer switching supplies for phones give off a very stable 5V. The same type of supply is used for iPhones/iPods, Blackberrys and most other cell phones. In fact any device that charges via USB connector uses that kind of power supply/charger. I have a few around my desk for the various gadgets I own and they make great Arduino power supplies.
You can tell the difference between the old transformer kind and new switchers by the weight. The switchers are very light and hollow feeling compared to a transformer of the same ratings.