Here are references and notes about using Git for FreeBSD development.
(editor's note: topics such as which source control system to use can tend to be religious in nature -- especially when it comes to the idea of migrating any and all FreeBSD repositories. Please keep this in mind as you read the following pages.)
Overview
A good introduction to Git was presented by UlrichSpoerlein at the Devsummit 2010 in Ottawa.
Some of the reasons why Git rocks can be found on the GitConversion page (by FlorentThoumie).
fabient@ has written up some strategies for working with the Gitorious mirror.
UlrichSpoerlein has also written up a description of his Git workflow. Another workflow is by JonathanAnderson.
PeterWemm, however, is pessimistic about the usability of Git as a replacement for CVS.
Mirrors
your.org hosts Git mirrors of various BSD projects, including FreeBSD src and ports trees.
There is also a FreeBSD project page on Github with two repositories: freebsd-head and freebsd-ports, but it does not include all of the Subversion branches, making it harder to e.g. diff against a stable branch.
The Gitorious mirror does include a limited set of SVN branches, but JonathanAnderson's opinion is, "for the sake of a couple hundred extra MB (an extra 40% or so), why not carry around the complete revision history of the project *and* all branches, including releases, vendor branches and user stuff?"