Just stumbled on mod_pubsub a combination of various clients and a mod_perl application for enabling publish subscribe two way interactions with browser clients over HTTP. The demo apps indicate that browser mileage will vary - but that is to be expected when using specs to their limits.
This open source project comes out of what looks like interesting work by Rohit Khare.

[UPDATE]
As Nikolaj informs me, I actually stumbled on mod_pubsub indirectly a few years back when KnowNow (the company sponsoring mod_pubsub) was featured in Release 1.0. I remember reading the description of the technology and thinking 50% vaporware and 50% brittle borwser hack. The brittleness I assume is still an issue, but no vapor is to be found.