I'm curious to hear about an iPhone 4 car charger as well.
I have an "older" Griffin PowerJolt charger that worked with the original iPhone, the 3G, and the 3GS. However, it will NOT charge my iPhone 4.
I've tried 2 different Apple USB cables and the black Griffin USB cable. When I plug it in, I hear/see the "charging" battery indicator on the screen, and it charges...
...for about :15 seconds. Then it stops charging. I don't get it.
Same thing happens to the wife's iPhone 4. Plug back in my 3GS, works great, no matter what cable I use.