Comet VS Ajax polling
Latency is not the only problem. COMET (long-polling) "saves" your traffic - when you use polling, you cannot know, if there were changes on the server, so some of the calls may be just a waste of traffic and resources (e.g., even if no one's chatting, you're making calls every 3-4 seconds). In case of COMET, you generally need one just call to get an update from the server (with 100% hit rate).
@Vasil: "you can keep millions of opened connection"
Take a look to this problem
And regarding the false superiority of NIO over IO.
The iobound article just shows IO outperforms NIO and yes IO goes out of memory before NIO (by the way, the author has not tried to reduce the stack memory with a lower value).