A recent discussion in the comments of Comet is always better than polling brought up the point of the 2-connection limit defined in the HTTP specification, and its enforcement by Internet Explorer.
Stepping back for a moment and defining the problem, you are allowed only two simultaneous connections between a client and a web server, making things especially difficult for long-polling and the forever-frame technique, the two staple Comet techniques. Mozilla Firefox by default provides web applications with 8 connections by default, which is enough for Comet developers.
If you really only have two connections, and one dedicated to the Comet connection, you do have a problem. That said, the solution is very straightforward: subdomains! Internet Explorer treats the following four domains as unique and distinct in its calculation of number of connections allowed between client and server:
One note: we usually limit ourselves to four subdomains, as we find that an eight connection total limit exists in certain situations (such as Mozilla Firefox browsers using the default configuration). This is a hack that works with great success, much like many of the other common hacks in widespread use on the web.