Yeah totally, we were thinking of setting up a geo fence around the airport and automatically sending a text with their location or notifying the person being picked up that their ride has arrived.
Geo-fencing (at least on my 4S using location-based reminders a la Siri) was an intense battery drain. As cool as it is, I don't use it often for that reason.
Not sure if things have changed since launch, but keep that in mind - if you do implement, maybe make it easy to turn it on/off?
You should also check out Glympse which is what I use for this kind of thing. It is sufficiently real time and accurate that the people I'm picking up surprise me at my car as I park. It also helps when you get unexpectedly delayed by traffic.
