So idnar just turned me on to darcs-fast-export, which can be used with git-fast-import to quickly convert a repository from darcs to git. I've been using Git more and more in the last few months, and I'm growing quite fond of it. Tahoe is managed in darcs, and I've been using a private Git mirror to manage the several dozen feature branches that I work on at any given moment. I wanted to make a more-official mirror that would be reasonable to publish on GitHub.
I had to patch the darcs-fast-export script a little bit, one because our darcs repository happens to have some bad (non-UTF8) characters in some old patches (before darcs started rejecting those), and two because I wanted to preserve our tag names (like "allmydata-tahoe-1.4.1", and darcs-fast-export was squashing the hyphens down to underscores).
Tahoe has about 4000 patches. darcs-fast-export started doing about 170ms/patch (20 patches per second), and towards the end of the job is slowing to about 1.1s/patch. In contrast, when I first tried the conversion with tailor, the "darcs pull" operation was taking about 20 seconds per patch. Tailor finished after 13.5 hours. darcs-fast-export took 42 minutes.
darcs-fast-export also takes care of incremental updates, so I can update the mirror later as more darcs patches arrive. It also suggests that it can be used bidirectionally. I might start using this to move my git patches back into trunk.