Chandu Thota (of BlogMap fame) has a link to an MSDN article on implementing a Geofence with MapPoint Location Server. "What's a Geofence?" I hear you cry. I'm glad someone asked.

From the article:

A geofence is a geographical region that you define and use to trigger an event when a user enters the region. For example, you can create an application that sends an Short Message Service (SMS) message about a current sales promotion to provisioned users when they come within a one-mile radius of your store location.

Map with a Geofence

Very cool stuff.