Personally, I prefer Juggernaut (http://juggernaut.rubyforge.org/) for this. Sure, it's Flash, but it just seems like a bit less of a hack than long-polling.
http://cometdaily.com/2008/09/30/why-flash-must-adopt-comet/
I am using it on port 443 (https).
.... and there goes half your potential users.
That aside, I also can't run flash on my iphone, or my blackberry.
Javascript on the other hand is supported nativity across the board.
The advantages/disadvantages listed on the juggernaut website seem fishy to me.
1) it's not hack
2) I don't know of js crashing your browser *more* then flash
3) Any browser that supports flash supports JS
4) You can use http over any port as well
With Comet you get the added benefit (which is small or large depending on your implementation) of http caching and you remove reliance on 3rd party libraries.Looks like I just rewrote what was previously mentioned in the articles about juggernaut