I've been a long time fan of outdoor security being handled by the four legged variety. In our case, a pair of boxers who will let me know if they have guests at night and are pretty good at convincing people that they are meaner than they are.

Barring that, take a look at X10. They have been around for a long time (I know I bought a camera from them back in 1996 or 1997 to watch my car parking spot in an apartment complex I lived in back then). They are pretty inexpensive and have a whole bunch of things to choose from - from the passive cameras to complete systems with lights and alarms. Something like this driveway alert system might be right up your alley:

http://www.x10.com/promotions/sc546_...veway_hmp.html

Motion sensor and alarm to let you know when the sensor gets tripped. Wireless and battery powered.