Through Sam Ruby, a pointer to mod_speedyfeed, an Apache 2 module that allows feedreading clients to only download the entries that have changed since the last access instead of the entire RSS file. This could cut the transfer amount significantly for many syndication sources. For now, it just works for Atom, but I don't see anything about it that couldn't work for RSS as well. The module is triggered by a header that the he client adds to the request like so:

GET /asdf/atom.xml HTTP/1.1
Host: asdf.blogs.com
If-Modified-Since: Fri, 17 Sep 2004 00:18:36 GMT
A-IM: feed