So first what on earth (pun intended) is a GeoFence?   Have a look at  what I've been reading -

http://msdn2.microsoft.com/en-us/library/ms955270.aspx

and also looking at the Virtual Earth SDK

http://dev.live.com/virtualearth/sdk/

Particularly interesting on plotting polygons and stuff.

So I'm having a go at building an Windows Mobile application that will automatically detect when I cross into GeoFence boundaries.   I'm thinking raising events etc.  as you cross into them.  Hmmm lots of lovely code..   Will keep you posted.

I wonder how you weather proof a GeoFence?