To elaborate on the possible uses of Reperion to integrate with Plazes or Twitter...
Plazes is a map-based social networking site that asks where you are and what you're doing. So, if you craft an SMS to their sms number from the user's cell number including a message (like the messages on reperion) and a location, it will update the user's profile. However, you also have to use a geocoder to get the location, as coordinates aren't taken. The protocol for Plazes is 'message @ location' ie drinking beer in Washington DC.
Twitter is a general update feed, not specifically map based. It's very similar, but it just needs a txt or im to go to a particular address. This also requires crafting an sms from the user and geocoding the results... See Mologogo's Molotwit for the concept.
Hope that Helps,
Dave