Further to my post about a protocol for Twitter posts, you can also triple-tag blog posts, Flickr images and similar web utterances, which refer to a specific twitter post (or status) like this: twitter:status=1975532392 – and this post is tagged with that!
[Update: See also my Flickr screenshot of a Twitter post, triple tagged with #twitter:status=1828036334 to reference the same post.]
Further discussion at microsyntax.pbworks.com/Triple-Tags.
You can also tag posts, etc., with user IDs, thus: twitter:user=pigsonthewing