This was way more difficult that I thought it would be, but I have conquered. I may be the only person in the world who even cares about doing this, but I'm writing it here so I don't forget.
I think it's hard because both the revision control tools, and the conversion tool are all still pretty new and in a state of flux. I finally got it to work using bzr version 0.15.0, git version 1.5.2.3, and tailor version 0.9.29. Both bzr and git are just what I had on my Ubuntu Feisty Fawn box that I keep up to date with backports and everything. Tailor version 0.9.26 is in the feisty repositories, but it most definitely didn't work for this. I grabbed the tarball for the latest source from here. You can just untar it and run it right from there.
OK, now, once you have tailor, to make this work, you need a tailor configuration file. Just put it anywhere and name it whatever you like. Mine looked a little like this:
[DEFAULT]
verbose = True
patch-name-format = ""
[project]
source = bzr:source
target = git:target
start-revision = INITIAL
root-directory = /homes/brmu/testing/fw_tests
state-file = tailor.state
[bzr:source]
repository = /homes/brmu/lnx/fw_tests
subdir = fw_tests-bzr
[git:target]
subdir = fw_tests-git
The paths for root-directory and the subdirs are just what I needed for my particular project. This tells tailor to grab the bzr repo from /homes/brmu/lnx/fw_tests, and then (I'm not sure exactly why it makes a copy) copy it to /homes/brmu/testing/fw_tests/fw_tests-bzr. Then it creates a git repository in /homes/brmu/testing/fw_tests-git and does the migration/conversion from bzr to git using these two new directories. I already had the /homes/brmu/testing/fw_tests directory created. Once you have this configuration file set up you can run tailor like this:
/path/to/latest/tailor -D -c tailor.conf
tailor.conf is the configuration file, of course. I found the best documentation for tailor in the README file with the source code. There is a wiki and a manpage, but those were both not as helpful.