Launchpad already supports git but there are still many active bzr
If you would like to migrate some of them to git I'd like to suggest
taking a look at bzr-git-mass-convert  based on bzr fast-export
(verifying the result with git-remote-bzr). It is a simple tool for
merging multiple bzr branches to a single git repository, set up for
pushing it back to Launchpad.
We (at the Foundations Team) use it for migrating our repositories and
there is also a wiki  page for tracking the migration schedule 
of popular repositories.
For example we plan migrating livecd-rootfs on December 12, please get
your open MP's merged by then to avoid the extra work of resubmitting
them against the new git repo.
There is also a new tracker to list packages still maintained in bzr,
grouped by the teams .
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