Kvdb:
The plan is that the device will connect to every hotspot it passes.
Do you want to know where the car HAS BEEN while it was away from open hotspots or do you only need to know the location when it is in range of an open hotspot? If you want to know the path the car took between hotspots you can use the nifty new GPS from adafruit: Adafruit Ultimate GPS Breakout - 66 channel w/10 Hz updates [PA1616S] : ID 746 : Adafruit Industries, Unique & fun DIY electronics and kits It can log positions every 15 seconds for 16 hours into internal FLASH while your Arduino sleeps.
If you can get an old Linksys WGA54G Game Adapter it will automatically connect to the strongest open WiFi signal it can find. You'd need a Ethernet Shield to talk to it.