Ok, so like I said earlier, it only took a few hours of playing around to get the first version of mod_speedyfeed working and out the door.
Get it while it's hot: http://electricjellyfish.net/garrett/mod_speedyfeed/mod_speedyfeed-0.02.tar.gz
You'll also need a patched version of Apache2 to get support for the new 226 status code, grab the patch from the Apache Bugzilla.
What does it do?
In short, it allows you to only send new entries in your Atom feeds down to the clients. The client program adds a few HTTP headers (a If-Modified-Since to tell you what the last time they got was and an A-IM that indicates you support the 'feed' IM) and things just magically work.
Best of all, the content that's sent down, while smaller, remains valid Atom XML, so no real change is needed on the client side other than sending the new headers.
All you need to do on the server side is compile and install the module, it works as a filter so any content that's served up with an application/atom+xml content type is automagically effected.
(The first release is 0.02, not 0.01, since I naturally found a show-stoppingly bad bug at the last second...)