Its funny but this is one of the first things I tried when I got my Droid 2, just to save money on car charger. I have tried my Blackberry Tour car charger, home charger & usb which was already hooked up to my pc.
The car charger hooked up during an hour long ride to work, and hour long ride home did not change the battery % at all. It actually took some moving it to get it to show the lightning bolt indicator on the phone. Home charger was same way, although it did charge, but only to 70% overnight. USB cable produced same as car charger.
Headed to Verizon today to get a bundle with car charger & screen protectors. The battery life is horrific on this phone, need a charger at home, work and car, but DAMNNNN i love this phone!