In Smart Home Hacks, Michael Ferguson (creator of the Mac home automation application XTension) contributed "Hack 54 - Monitor Your Driveway" which describes his techniques for knowing when a vehicle has entered his property and when it has arrived at the house. It uses some simple, but useful, scripting along with motion detectors and a photo-electric beam.
Neil Cherry, on his Linux home automation resource list, pointed me towards an interesting alternative approach by Fred Stelter, as published in Linux Journal. Fred's techniques uses a serial I/O box, a magnetic sensor, and some clever Python scripting.
When it comes to home automation, knowing multiple approaches allows you to choose (or more likely, combine) what works for you.