I think there are amazing opportunities in Twitter's new annotation feature.
I thought I would share some brief thoughts that might be of interest. The schema of LinkedData RDF, underpinnings of the emerging Semantic Web, provide a sound basis of the sorts of format & protocol standards necessary for the APIs you are proposing. The vision of Semantic Web is occasionally dismissed as long on dream and short on practice. However, early implementations are gaining significant traction and offering substantive value to early adopters. Thompson Reuters, no slouch in the business of information, was quite visionary in acquiring the now @OpenCalais service. The mostly free service is currently identifying semantic entities in over 5 million documents submitted to it per day. @zemanta is also finding a significant user base among bloggers for its related services. I am awestruck by the potential of developers applying Linked Data / Semantic Web schema and using RDF in Twitter annotations. It would massively scale search effectiveness and distribution opportunities while allowing sophisticated on-the-fly analysis of Twitter's firehose & other feeds. The Twitter ecosystem is particularly suited to applying principles of the Semantic Web in that machine interpolated meaning could be continually refined the more humans tweet & retweet about the same and related topics; coupled with author, location, hashes, platform, and temporal information. Twitter annotations may well be a turning point in the early practical application of Tim Berners-Lee’s 1999 “dream for the Web [in which computers] become capable of analyzing all the data on the Web – the content, links, and transactions between people and computers.” It is all about the metadata! -- Subscription settings: http://groups.google.com/group/twitter-development-talk/subscribe?hl=en