Has this been done yet?:

http://bikehugger.com/2009/09/charge-your-ipod-with-your-bik.html

Sadly it appears to need a Dynohub though 🙁
But the comments suggest various alternatives that may prove useful. Ideal for those that complain they don't get enough battery life to use their smartphone as a GPS.

That looks significantly nicer than the cheap & nasty one we have at work, which apparently has a tendency to kill phones, I've never dared plug my own phone into that.

Personally I'd really like one without a battery in, as I'm guessing the battery on the dynamo device will wear out after a while, but I don't know if it's possible to nicely regulate the dynamo current/voltage without using a battery in there?

Joe