It will depend on what brand of car charger you bought. I have found that most (USB) car chargers are rated the same as a PC USB port, 500mA. You can find in
some stores, a high capacity high amp car charger - and most of these put out the 1000mA that your factory wall charger do.
I have also seen a sort of multiplexer for USB ports on a notebook before. It occupies 2 of your standard USB ports, but gives you the combined output amperage of both through one port. Only one port is used for data, the other is just tapped into the power supply of the second port. Might be something we could all be interested in soon with the Pro.
***EDIT***
Heres one.
http://www.pccables.com/70650.htm