Have you ever interrupted an HTTP download and then restarted it later and had it pick up where it left off? That little bit of magic is the result of RFC3229: Delta encoding in HTTP. This is useful for more than just resuming downloads. Jon Udell, for example, uses this trick to access a small part of an MP3 file and its also the secret sauce in mod_speedyfeed that I mentioned yesterday.
This article by Bob Wyman that outlines the particulars. In an interesting adaptation, while RFC3229 has always been about byte ranges, but the mod_speedyfeed module uses RFC3229 with item ranges. The bottom line: is there really isn't anything about this that would keep it from working with RSS 2.0 as well as Atom.