Let me start off saying,  this isn't an original idea...   

But now that we're getting more and more mobile devices with built in GPS's or ability to triangulate their location.

I'd like to be able to post a text message not just to a person but to a location.    i.e I'd like to post a restaurant menu in space outside the restaurant so that I can browse it on my device when I get near.

Similarly, I'd like to have my agenda arrive to me when I pass a certain part of the railway line on my way to work.

Imagine what would happen,  if you used this to alert all motorists to accidents this way (or would it cause lots ?).    Anyway,    surely this is a perfect little Windows Mobile app. to be written.    Time to write some code....