BT (yes, British Telecom) have launched a Google Maps Mashup that appears to be what can only be described as a bit like a "GeoTwitter". Needless to say it's not called GeoTwitter - it's called Tell'M
Basically, you post a message and it appears on a Google Map. The messages at the moment are mostly "hello" messages, but it seems like the site is mostly designed for quick anniversary, birthday, births, graduations, etc messages. I guess it's yet another nail in the newspaper classifieds coffin.