<p>It's a simple case of a javascript that continuously asks "are there yet?" Like a four year old on a car drive.. But, much like parents, if you do this too often or, with too many kids at once, the server will buckle under pressure..</p>

<p>How do you solve the issue of having a webpage that looks for new content in the order of every 5 seconds and that allows for a larger number of visitors?</p>

stackoverflow does it some way, don't know how though.
The more standard way would indeed be the javascript that looks for new content every few seconds.
A more advanced way would use a
push-like
technique, by using
Comet
techniques (long-polling and such). There's a lot of interesting stuff under that link.
I'm still waiting for a good opportunity to use it myself...
Oh, and here's a link from stackoverflow about it:
Is there some way to PUSH data from web server to browser?