Seems that people are talking about using RSS as a blog export format again (I think this Newsforge article kicked it off). Dave Winer did this in November 2002 - then Brent Simmons did a Mac app to view the archives, and I did a Windows one (homepage).
These days, the Movable Type export format is what everyone uses to send their blog data and comments around. It seems that most blogs can generate it,
Danny Ayers pops up in the comments and suggests using RDF instead (surprise!) or perhaps Atom or HTML+microformats.
I like the microformat approach, but I can imagine that it would confuse the hell out of implementers, and if you're trying to replace the MT export format with something more extensible, it's going to have to be dead easy to implement. I'd go for the single-gigantic-feed approach, including comments and trackbacks as well. This is how the MT export format works, and it works fine. Either that, or require everything to be under a URL somewhere on the web, and force importers to download it piece by piece.