How do I build a bicycle powered phone charger?

My bicycle has a 6v 3.5w AC dynamo hub, and a phone which charges from USB (5v). There are many LED/halogen headlamps with NiMH/LiPoly battery packs which are 6v DC. What I’d like to do is rectify the dynamo’s AC to DC and charge the battery from the dynamo. Once the battery is being charged by the hub I will not only have a headlight, but I could also easily convert the 6v AC to 5v and use a USB outlet to charge my iPhone. I’ve been wanting to do this project for months now, but I need help with the details of the electronics. Please advise about which battery packs will work best, and how I’d wire them up to the hub.