Twitter enables CLIs for the masses

Twitter is simply message passing
Up to this point, most of the messages on Twitter have been from human senders to human listeners. However, there is no reason that people should be the only agents who send and receive messages and in fact we are seeing the rise of services that replace humans on one or both of these ends.

Messages have lots of context
Services that receive messages on Twitter need to be able to parse the message for meaningful context and perform one or more actions based upon the content of the message. The more specific the service, the more specific the message needs to be. For example, search.twitter.com is a service which listens to every message and indexes the content of every message so that it is searchable. 'Trends' is a service built on top of this index that notices which terms are experiencing accelerations.

Message syntax conventions
It's not too difficult to extrapolate the growing usage of Twitter as a 'universal' messaging platform as certain conventions become widespread (e.g. beginning a message with a twitter id means a "reply", using a '#' symbol in front of a term indicates a tag) and new conventions are adopted. It is not too hard to imagine how even something like the unix pipe symbol ('|') might be utilized in Twitter to string messages along (e.g. "@nwa nyc tues am | @cal @bob 2pm") might use a service from Northwest Airlines to schedule a flight for you on Tuesday morning. Northwest might then pass the result of their service (perhaps a confirmation code, flight number and time) to the next service in line -- a calendar. That service would add the flight info to your calendar as well as send an invitation to @bob to meet at 2pm on that day.

Conclusion
Twitter's popularity growth has certainly benefited from people using it as a way to pass messages among themselves but the service will become indispensable as more people discover useful services which can be invoked by a simple message yet which can perform powerful actions on the user's behalf.