(I wrote the Conditional GET guide linked above)
Conditional GET was a good first start to the problem. Most news aggregators and content-management systems are moving quickly to support this (even Movable Type, which already generates static files, has been patched to make sure that during updates, it won't change the Last-Modified date on files that have changed.) I checked my access logs, and the day before I posted the article above, only 10% of requests for my RSS files were getting "304 Not Modified" responses. Ten days later, that was up to 44%. I suspect that figure will improve, many aggregator users haven't upgraded yet.
(My article didn't have much to do with the adoption, by the way, it was just a convenient summary of what lots of people were saying)
Gzip Content-Encoding (compressing files during transfer) can save more bandwidth, but it requires additional client/server negotiation, and most of us aren't friendly enough with our web-hosts to get them to install arbitrary Apache modules.
NNTP has the advantage that it's an existing protocol, but it has the disadvantage that the existing news infrastructure is totally unsuitable for secure, noise-free article distribution.
Dave Winer has implemented a server-level cache for RSS feeds, which works with his Radio Userland client-base. This is an RSS concentrator, it focuses the bandwidth costs on the cache rather than on the individual RSS-producing sites. Which is fine if, like Userland, you are charging your users for the privilege of access, but is probably less workable as a general solution.
Instant messaging is probably the most workable solution. The correct fix is to change the mechanism for update notification so that it is server-triggered instead of client triggered, removing the need for polling entirely, and ensure delivery of the update information is multicast rather than pointcast. It wouldn't be hard to write a module for Jabber that would allow anyone to subscribe to a newsfeed, so that when a site updates, it automagically pushes the details of the updated article to all subscribed clients via the network of Jabber servers.
An additional advantage of the Jabber-based system is that it allows immediate notification as news comes in, rather than having to wait an hour for your RSS aggregator to cycle.
The more I learn about the Internet, the more amazed I am that it works at all.