I've had the TP overwhelm an AC charger, too. A part of that issue is heat, too, I am sure. I just got a car charger for mine, and it charges at the normal ~900 ma, ran Sprint Navigation for about an hour, sitting idle the majority of the time. When I plugged it in, it was fully charged. When I unplugged it, it was at about 73%.

In my tests charging it with the computer instead of the wall charger, while there was a slight difference, it really wasn't worth it. I found it more effective to let it "fully charge," unplug it for 5 minutes or so, and plug it back in. Charges faster, lasts longer. I suspect the heat has something to do with the voltage reading high, it thinks its fully charged, then when it cools back to a normal temperature, you're really in the 90% range.