AJAX polling alternatives

I recently started investigating the AJAX framework and have a couple questions/concerns. I think the greatest possible benefit of AJAX could be in streaming data. That is, only sending out new XML to the client when there is some state change in the database. This unfortunately seems impossible because you cant listen for an uninitiated server message from within the browser.
So the only alternatives to polling are leaving persistent http connections open between client and server which eats up server resources. My problem with polling is that my users are likely to pull less data just navigating through a synchronous front end than they would use if all the data was being refreshed periodically through polling. So only real benefit is that the client app runs slightly faster because it already has [possible expired] data stored on the front end.
Does anyone have any clever ideas about this? My one thought is to add a 'last-update' field in the user table and an additional state_change table for all other types of data on the site. Instead of polling asynchronously, i would only pole when a user naviages to a different page, but instead of pulling all the data, i would just do a quick search of the last_update date, and if my front end cached data is still the latest version, do nothing else. Only if last_update is new would i reload the XML data.
Anyway, if anyone else has any thoughts, suggestions, or alternatives, i would definately like to hear. thanks a lot.
PHP/MySQL

Have you taken a look at Comet? This is trying to solve this issue.
http://www.ajaxian.com/archives/comet-a-new-approach-to-ajax-applications
-Larry

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