I think they're using comet for the requests so while the activity may be high when people are typing, during breaks it should be very low as opposed to ajax polling.

I do think that real-time is really needed for many applications. The few times I've tried collaborating on a google doc, I've given up because the lag introduced the possibility of overwriting each others edits. That and just a general annoyance with saying "Do you see it yet?" over and over.

Well that's a stupid way to handle it... keep the socket open... read about Comet. http://en.wikipedia.org/wiki/Comet_(programming)

Are you being sarcastic? Does anyone know what comet is? Please explain exactly how you can SEND multiple packets over time from browser to server without sending more HTTP requests, and without using any java applet/flash.

It's a shame people seem to have started saying "use comet" without understanding what it means. Did you read that link?

Connection: Keep-Alive

Yes, you can specify keep-alive (Although some browsers refuse to do it in an XHR - Opera for one). However, you still send the full glut of HTTP headers every single request.

True, you still need the headers, what you don't need is a new connection every key stroke. Establishing a new TCP connection is a relatively high cost network item (between all the Acks and Syns you need 3 messages have to go back and forth for it to establish), keep the connection alive, the stuff will run a lot faster.

You can null out the default headers on a XHR so they don't get sent.

Without any java applets or flash?