There's a way to do this using "pushlet" technology, but it's rather complicated (involving Frames, JavaScript, and persistent browser connections).

The idea behind a pushlet is that a hidden frame keeps a long-running HTTP GET going to a server. The server dribbles out JavaScript commands, which get interpreted and executed by the browser as they arrive. These commands invoke JavaScript routines that are embedded in either a parent window or in another frame. Typically, a side-effect of this invocation is to refresh part or all of some visible frame.

This collaboration requires that the server-side keep a long-running connection. Most off-the-shelf web servers will time connections out.

KnowNow, Inc. pioneered this technology (though they didn't invent it). They also recently started a SourceForge project to release some of their code publicly.

A google search on "pushlet" might turn up more usable info.