Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD maintain AT MOST 2 connections with any server or proxy.
is mentioned. A work around given by IE for this issue is here. But this cannot be used in our case as we can't ask our clients to change their registry settings.
Instead of IFRAMEs you could have DIVs loaded asynchronously using the XMLHttpRequest object. In IE 6, at least, this ignores the "2 persistent connections" issue somehow. Of course that kind of loophole might be closed in the next patch, and to use XMLHttpRequest your clients would have to have ActiveX enabled in IE. Note that on Mozilla browsers the max persistent connections is defaulted to 2 and that does affect XMLHttpRequest requests. But this can be changed through about:config without touching the registry.