What's the most CPU economical for location "fence"

[dominoid]

What is the most CPU friendly way to setup a location radius fence? I want to do something when a player comes within a certain radius of an in-world object or location. I know I can use OnUpdate, but that should be avoided if possible right? So . . . is there another way? If not, what is the best way to throttle OnUpdate so its not too intensive?

TIA