I have a 3rd generation iPod Nano. It detects it's connected to but refuses to charge from a generic car->USB charge adapter, but I didn't fancy buying an adapter cable or yet another charger specifically for the iPod, so I modified one I already had.
There's a chance that this could work for other USB devices (maybe an iPhone too, but I don't have one), but there's also a chance that this could fry your device, car, or perhaps eat all of the cheese in the house
. If you're not confident, or not competent, you probably don't want to try this. I accept no responsibility for any bad things happening.
If you're vaguely interested in stuff like this, you should probably check out ladyada's MintyBoost!
kit, which helped prod me towards actually doing this.