It is obvious, that the question "where are you" is one of the most often asked during the communications. 600 billion text messages per year in the US ask "where are you?" - as per Location Business Summit 2010 data. A huge amount of mobile services is actually being built around this question so their main feature is user's location exchange. In the most cases it is implemented as the ability for the mobile user (mobile phone owner) write down own location info in the some special place (special mobile application). But it means of course, that user must be registered in this service (download some special application). And even more important - everyone who needs this information must use the same service too.
Typically we have now two models for location sharing in services. At the first hand is passive location monitoring and future access to the accumulated data trough some API. It is Google Latitude for example. Possible problems are privacy - some third party tool is constantly monitoring my location and what is more important - saves it on the some external server as well as the shorted life for handset's battery.
Another popular model for location sharing is check-in procedure in the various forms. It could be an active (e.g. Foursquare), when user directly sets his/her current location or passive (e.g. Twitter) when location info could be added to the current message (status). But here we can see not only privacy issue - all my friends/followers can see my location but also a noise issue - my location info could be actually interested only for the physical friends. For the majority of followers my location info (e.g. Foursquare status in Twitter time line) is just a noise.
Our idea of the signed geo messages service (geo mail, geo sms) based on the adding user's location info to the standard messages like SMS or email. Just as a signature. So with this service for telling somebody "where I am" it would be just enough to send him/her a message. And your partner does not need to use any additional service in order to get information about your location. He/she will simply read SMS or email.
It is obviously peer to peer sharing and does not require any social network. Our location signature has got a form of the link to some mobile map with the marker at the shared location. And what is important here - the map itself has no information about the sender and recipient. That information exists only in the message itself. The map (marker) has no information about the creator for example. That is all about privacy.
There are several implementations for this approach: mobile web mashups (HTML5), mobile web widgets, Java Card applets for legacy phones, JME midlet for mobile Java phones as well as solutions for web applications.
You can see here a presentation for this approach from ICUMT-2010 - Geo Messages approach.