create an event within the twitterstream.

"anyone interested in a lunch tweetup at the waterfront at 12:30pm?"

people follow the event bot. the event bot parses event data out of the tweet.
probably use markers to help the bot. &=event *=location

"anyone interested in a &lunch tweetup at the *waterfront at 12:30pm?"

locations and events are unique to the user so names like lunch can be reused.

the event is stored in an event db (calagator?). enhanced twitter clients will poll for event data to display possible activity options. activities that have been initiated by twitter friends.