I have a Griffin USB wall charger I use to change my Sansa MP3 players, but it didn't work with my PRS650. The specs on this say it is 5v, 1000mA.
Neither did my Belkin USB car charger - the PRS650 saw it as USB, but didn't kick over to charge mode. This one is labeled 5v, 1A output with a 12v input. The cig lighter port I had it plugged into would have been putting out a little over 12v, because that's what motorcycles do, but I wouldn't think that would matter.
The USB port on the Alpine stereo in my truck didn't even look like USB to it, it was like it wasn't plugged in at all. I have no idea what specs that port puts out, but the stereo was on so you'd think it was live.
If the chargers for the Kindle 3 work, maybe this one will do the trick:
http://www.amazon.com/CrazyOnDigital.../dp/B004165BIU
I'll order it and see.