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.
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.