I recently did some homework on this same topic and can provide you with a little amplifying info. What you're looking for is a charger that provides a "dedicated charging port" which is currently defined in the Battery Charging Specification Rev 1.2 found here:
http://www.usb.org/developers/devclass_docs/BCv1.2_011912.zip.
There are two important items to look for: rated output of the charger (e.g. 5V / 1.0A) AND that the data pins (D+ and D-) have a MAXIMUM resistance of 200ohms (RDCHG_DAT) between them. Note that a dead short is perfectly acceptable.
However, most of the adapters I looked at don't provide this kind of information in the specs on retailers' websites. In a lot of cases you will have to resort to reading reviews to find out whether the charger will give you the full rated power. I can tell you from experience that this charger I recently bought does show up as an AC source when charging my gnex:
1000mA Car Cigarette Powered USB Adapter/Charger (DC 12V/24V) - Free Shipping - DealExtreme. It did take a couple weeks for it to make it overseas.