From Mexico.purplecow.org
Cheap USB charging devices only provide a +5VDC rail. Nothing on the data lines - not connected. This works for a lot of things, but my Motorola Milestone and iPod won't charge because they expect to see some life on the data lines.
I had a quick Google and found a number of solutions, of which this was the simplest.
+5VDC | | +----+----+ \ \ 150k / / 50k \ \ / / USB D- --+ +-- USB D- \ \ 100k / / 50k \ \ / / +----+----+ | | GND
Of course, I didn't have any 50k or 150k resistors in my junque box, so I subbed 47ks in. Still works just fine. Both iPod and Milestone charge away nicely.
I've been told this won't work for the iPhone 3G or newer because they're _really_ fussy about the voltages they expect. I'm not too fussed since I'm unlikely to buy an iPhone.
Credit where credit is due: I looked at a quite a few ideas while poking around with this, but this link was the most obvious and clear: http://tzywen.com/modules.php?name=News&file=article&sid=683
(I'm stashing this note here because I don't trust forums to be here in the future.)