As I was twittering earlier in BART, I had to switch from the mobile Web twitter interface to the SMS interface. It suddenly occurred to me that just as I was able to choose which communication protocol (HTTP POST, SMS, voice-to-text: Twitterfone) and I suspect the reverse service exists as well) to post a message, my followers would be able to choose to read them using the reader of their choice (Twitterific, SMS, desktop Web, mobile Web).
This reminded of the old concept of Unified Messaging/Communications. Here is the definition from ZDNet:
The realtime redirection of a voice, text or e-mail message to the device closest to the intended recipient at any given time. For example, voice calls to desk phones could be routed to the user’s cellphone when required. E-mail intended for a desktop mailbox could be sent to the user’s PDA or turned into speech for a phone message.
Twitter is indeed pretty close to being the version 1.0 (beta – for its reliability issues) of Unified Communicator, and it probably achieved adoption where others failed or partly succeeded because 1) it limited the problem to 140 characters, a good common denominator for a variety of protocols, in particular SMS, and 2) APIs were exposed for 3rd parties to extend its capabilities.