I also had the problem of not being able to find a USB power supply that would charge my Sony PRS-650. I kept getting the "power supply not detected" message, even though I was using USB "bricks" up to 1 amp capacity.
After noting that the USB standard required pins 2 and 3 (The data pins) to be shorted, I came up with a fairly simple solution. All it requires is a cheap USB extension cable, a continuity tester, and a modicum of manual dexterity.
I stripped off the outer insulation of the USB extension, made a gap in the braid/foil shielding (I didn't cut the shielding, just pushed it off to one side) and exposed the 4 USB conductors. The data conductors on my cable were green and white, but your results may vary. I stripped the insulation from a small section of the green and white conductors, and shorted them out with a short length of copper wire, then insulated all the connections with electrical tape.
Plugging the modified extension cable into my USB "wall wart" and using the USB charger/data cable that came with the Sony Reader, I now got the charging light, the charging symbol (arrow pointing at battery in lower right corner) AND was able to read a book while the battery was charging. My USB supply was 500 ma - rated, but it worked like a charm. The same setup worked for my iPod and my Palm PDA.
Total cost (as I already had the USB supply) was about $2 for the USB extension cable. If you already have a couple of these kicking around, you can even save yourself that cost. And if you want to be able to use the cable for data, you could easily wire a switch to either short out the data leads (for charging) or not (for data transmission).
