Just found this one, nice little simple thing and by the looks of it will do the job. If you just need to charge something via USB this is the one. Most mobile phones and mp3 players let you do that now.
The Bicycle USB Charger allows the charging of electronic devices from a standard bicycle dynamo. Simply connect to a bottle or hub dynamo, attach to your bike, and plug in your mobile phone, mp3 player, GPS, or any other device able to charge via USB.
And at £25 it is probably the cheapest dyno hub charger around, unless you build it yourself.
LINK : bikeusbcharger.com