As of April 2009, the gitosis package in Ubuntu 9.04 Jaunty is broken; it fails with an error like so:
There are quite a few pages and mailing list messages that mention this. I only found one with a good hint toward a solution, which was that it is also a known issue on Debian. Following that lead, I got it working by grabbing newer packages from Debian Unstable:
sudo dpkg -i python-support_1.0.2_all.deb
sudo dpkg -i gitosis_0.2+20080825-14_all.deb
Use this at your own risk; your mileage may vary.