[UPDATED 1/23/2013] I am a moron.
git checkout <tag-name> -b <tag-name>
Git told me to name the branch after I typed in git checkout <tag-name>. Smarty pants. If you've already checkout the tag it says you are in a detached HEAD state, use git co -b <new-branch-name> which could have been combined with the checkout. Hence the command above. QED