We have this 3rd-party darcs-fastconvert tool that uses the darcs
library to support fast-{import,export}.  Note that this tool is
distinct from the darcs-fast-{import,export} script.  (cabal install
darcs-fastconvert)

Unfortunately, darcs-fastconvert doesn't yet have its own
maintainer/project (it grew out of darcs), so we'll have to be
responsible for darcs-fastconvert issues too on our tracker.

We were talking about Brian's darcs-git bridge on
http://irclog.perlgeek.de/darcs/2010-10-17#i_2920950 when the idea came
up that extending the darcs-fastconvert to support marksfile would help
darcs-foo bridges a lot.  Right now there is another darcs-fast-export
tool that provides this support, but apparently not to the extent we
have in mind

I'm taking the liberty of making Brian nosy on this ticket because I
think he'd be interested.

The one suggestion I'd make is to have the tool write out the marksfile
every 100 commits or so. In many of my migrations, I've run into bugs or
other problems and had to repeat hours of work because the tools only
wrote the marksfile at the very end, and they crashed before they got
that far.

Note that the latest version of darcs-fastconvert now supports 
marksfile.
Pretty exciting stuff for people interested in bridging.

http://lists.osuosl.org/pipermail/darcs-users/2010-November/025670.html