Mercurial 1.5 was released yesterday, and it includes support for "subrepos" -- like what is done in the Enzo-1.5 SVN repository.
This means that other hg repos can be included in a single, unified repository. Very cool, and we might be able to use this for things like the cookbook, the barn, the yt repo, and any other sub-projects of yt. hg includes svn support as well in the subrepos. :)