Bad idea. I have that adapter, and I use it for my ReplayTV. It's good for very specific uses, but in an environment like a car, it will SERIOUSLY cramp your style.
You will not be able to:
1. Connect to more then one network without reconfiguring it manually.
2. Map access points
3. Do just about anything.
If you're REALLY worried about taking up a USB port, invest $15 for a USB 2.0 hub, not $100 for an adapter that will leave you seriously disapointed.
This isn't a slam against the hardware, like I said, I have one, it's that there are SERIOUS limitations you encounter when you use this adapter for anything other then a fixed emplacement.