Lot of talk about Ajax being a pull (poll) only technique that has its limitations. This is true but there are also implications for using open http socket connections as well. I've made comments over there and quoted here:

Brent and I thought of using an open connection instead of a polling connection when thinking about scalability for BlogChat but decided against it. Although clients will be polling at some interval, the number of concurrent connections at any one time is manageable and works very well in a distributed webserver farm. With an open connection method like mod_pubsub, resources are held the whole time while a client is connected. If the server is overloaded all of the clients are affected and likely lost under severe circumstances. With a polling type of scenario one client may be affected for that instant it hits that particular server in the farm. Of course there are advantages and disadvantages to both methods that should be weighed depending on the application you are developing.

Rohit Khare weighs in about whats next after Ajax here. I find it a bit interesting the mention of KnowNow (which I won't like to now) since according to Phil Windley, Rohit is a founder of KnowNow.

Actually through Google cache result it seems mod_pubsub open source project was the basis of KnowNow. That info has been pulled off of here:

Dr. Rohit Khare. I am a graduate of Prof. Richard N. Taylor's software research
... the open-source Mod_PubSub project that eventually grew into KnowNow.

Oh well, I guess they didn't want anyone to know that fact.