Mobile and Location Based Wireless Presence

Presence is best described through the example of most Instant Messaging software. Simply put it is a way for software and systems to always know the availability of the users. This is also being extended to the wireless communications world with Push to Talk features on phones. I am thinking that it will need to used for any peer to peer application.

So, the question is how will this work. It especially becomes an interesting problem when the status of the peer is changing constantly, as would be in the case of location based peer to peer applications. It seems unreasonable to have the application constantly sending messages with a new status of the presence, but is that the way it gets done today? Is there a technology that enables presence?