One of the reasons why Twitter's audience has grown so quickly is because its open API has allowed a multitude of applications to be developed to support and enhance the Twitter-universe.
The basis of Twitter's popularity is very simple, elegant, and potentially addictive: send super short messages (maximum 140 characters) to groups of "followers" via web, SMS, or IM and receive the same messages from all of your "friends." Now, TechCrunch reports this morning that "a subtle upcoming change to Twitter's API" will allow for more advanced functionality.
Dave Winer writes that there is "no user-level functionality yet to report," but that he will keep us posted.
The idea will be that you can send a simple "command" message such as "weather" or "score" to a specific "follower" and that profile will generate information in response, i.e. "It's hailing, take cover" or "Your boys are getting smacked by 'Bama," or whatever.
Again, this simple premise may have yet another profound impact on Twitter's penetration. Particularly for the mobile set, Twitter has the potential to become a dominant communications tool, not only for receiving and transmitting idle chatter, but for collecting quick and relevant information as well.
Businesses will build atop this platform. Sports updates, stock quotes, specific news and information queries, restaurant and club reviews, album and record reviews (send the ISBN or UPC code and instantly receive a 1-10 score as voted on by users, let's say): the sky's really the limit, all focused upon short, simple, and fast. That's the killer app.