I remember seeing a wireless ethernet solution that had a transceiver unit that you simply plug an existing ethernet cable into - no need for proprietary cards, etc., but, the bandwidth was so small that they had to use thin client to get any real work done. When I say small, I mean under 1 Mbps. -which is still faster than serial, but if I'm not mistaken, I believe you can get better data rates with IR.

If you have to do it in car - 10baseT is good to 100 meters. You can always drag a cable to the car and roll it back up when you're finished...