Is it possible to create a Network Based Geofence using the .NET platform?
For example, is there a way where users can "opt in" to a service on their phone without installing anything, that would track when they enter and leave a geofenced area?
On the most primitive level, you need to implement point-in-polygon algorythm with regards to your current location in space. Certain specialized RDBMSes (such as PostGIS) have it already implemented via
CROSSING spatial SQL queries.