I bought a mains USB charger from ebay capable of supplying 1A (from arygroup) for my HTC Desire, and was surprised when it wouldn’t charge as fast as the original charger that had the same rating. I thought that maybe it was just a poor quality charger considering the price, but then I noticed that the original charger the phone realised it was on AC, but on USB with the new charger.
Reading about USB phone charging on Wikipedia it appeared that the data pins need to be shorted together to signify that it’s a charger rather than a USB data port. Without this, the phone will only take 500 mA. So, a quick adjustment with a soldering iron and the charger now works as intended. The difference in charging speed is very noticeable!
Please note if you decide to do this to yours, I take no responsibility for you damaging your charger or whatever you attach to it…
Here are photos of before and after: