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Ian merged my work today, so bzr-fastimport (which contains darcs-fast-export) from bzr now has it out of the box.
This feature is about the following use-case: if you wanted to import a darcs repo to (for example) git, then you had to 'darcs get' the darcs repo first, then import it to git. Now darcs-fast-export can do this without a 'darcs get' if the repo is available via http, reading much less files, resulting less data transfer. :)
(The trick behind it is that darcs-fast-export reads only the patches, while darcs get reads boths snapshots of the repo and the patches, so a darcs get takes much more time than a darcs-fast-export.)