HTTP Streaming - Ajax Patterns: "Stream server data in the response of a long-lived HTTP connection. Most web services do some processing, send back a response, and immediately exit. But in this pattern, they keep the connection open by running a long loop. The server script uses event registration or some other technique to detect any state changes. As soon as a state change occurs, it pushes new data to the outgoing stream and flushes it, but doesn't actually close it. Meanwhile, the browser must ensure the user-interface reflects the new data."
More information on XML-PUSH technology.
Realtime on Rails
I remember the first time I heard about this technology was on the Netscape DevEdge site - which used to be an excellent resource - the good news is a mirror is being restored I believe that this was the article.