I'm actively looking into this at the moment. Beware, many of the older designs which worked fine on the original iPhone and 3G won't charge an iPhone 3GS and I believe iPhone 4 is the same.
Not only does the charger need to put some resistors onto the USB data lines to signal to the iPhone that it can take a load of current, but also (I've read) the charger must be capable of supplying 1 Amp. Since USB specification is 500mA max this is a bit cheeky but Apple will just do what they like.
Some models to Google are as follows, but I haven't confirmed whether each of them will charge a 3GS.
MiLi Power Crystal
Kensington Battery Pack And Charger For iPhone And iPod
Veho VCC-A007-PBP Pebble 5000mAH Battery
Proporta USB Turbocharger 3400
Just Mobile PP-08 Gum Pro
Ted Baker (Proporta rebranded I think)
PINES Rechargeable Backup Battery Pack
I'm coming to the conclusion that they're all way too expensive for what they are and really I wanted something that would charge from AA batteries anyway. All the AA chargers I've found will NOT charge a 3GS, so I'm going to build my own. But if you find something else that'll save me the bother, please post here
