For those who don't read
debian-devel-announce:
the Alioth team, with help from John Goerzen, has set up an
infrastruct^W^Wa couple of scripts on Alioth, so that it's now hosting
Mercurial repositories on hg.debian.org.
Please read the Alioth/Hg page on the Debian wiki for details, but don't expect many surprises: it'll work mostly the same way as the other VCSes hosted on Alioth.