http://www.grouplens.org/papers/pdf/ludford-chi2006.pdf

This looks alot like Place-Its. It seems like it has not really gotten off the ground yet.

Basically it sends a text when you are in the vicinity of a place where somethign on your to-do list can be done.

There are social applications as well, here is one example presented at the Microsoft Social Computing Symposium 2005.

"In PlaceMail, a user’s social network could be leveraged in fun and useful ways. For example, you’re working in your office and want a beverage from Starbuck’s but are too busy to leave. You associate a note with the nearby Starbuck’s and broadcast it to your social network. Soon, one of your colleagues appears at your door with a chai from Starbuck’s for you. He saw your note (asking any nearby buddies to get a chai for you) as he was walking near the coffee shop."

PlaceMail determines your location by using the built-in GPS on phones. It seems like you also have to enter a list of the places where your tasks often need to be done.

I don't know if this ever got going. It seems like a pain in the ass to set up and who has phones that do this? For this reason, I I am not sure what the intended audience was.