It wouldn't be difficult for a Real Estate company to do this. They could have downloadable waypoint (lat and long) for each house on their website. The waypoint would download directly to your GPS. This is very common today for a variety of GPS related activities. The waypoints could be displayed on your GPS's map so you would know where the houses are in respect to your current location.

I'm surprised nobody has thought of this yet.

Forget the expensive GPS. Not that it wouldn't work, but how about tuning in am radio station that would start play a recording when you come near a home for sale and would describe it. This system is used in some theme parks as you enter for information.

In response to Michael's idea, its already been implimented, though the signal isnt anything too strong. There is a yard sign out front and it tells you a station to tune into, inside the house is a low-strength FM or AM station.

I experienced it once. Pull into the driveway (as to not block the road) of the house you are investigating, and a repeating recording describing the house is playing from some station within the residence.