While looking at tweets for some months now I have a theory that tweets encapsulate some meta data in them. Inside the tweet there is already data about the time when it has been posted, information about the application with which it has been posted and from whom and maybe in reply to whom. Tweets usually show up in RSS feeds and thus can be displayed on other sites, like I have done with mine at this blog and on Livejournal.com. On the first site all @replies are visible although I do not like that, on the latter these are not visible. Also on Facebook only my regular tweets are showing, though retweets do show up there and I delete them manually from my feed.
There are a few and maybe many accounts on Twitter that use RSS feeds only and thus retweet tweets of other users. For example accounts like @haikutwaiku or @needsex are just collections of RSS feeds of other people and thus violating their copyright. If there is such a thing like copyright on Twitter. If I would search for a certain keyword the retweets of these accounts do not show up and therefore I get the idea there must be some extra information encapsulated in these tweets. When I have my ideas more clearly I will contact the people behind Twitter about this.
Other accounts I find interesting are collections of P2000 announcements from fire fighters that are taking place in Amsterdam or other major Dutch cities. These are shown on for example @p2khaaglanden, @p2kgooi, @p2kamsterdam, @p2kflevoland, @p2krotterdam and @p2kutrecht. These accounts do not use RSS feeds though, but a combination of PWD and ActivePerl.