From http://www.somethingorothersoft.com/2012/05/22/pulling-github-pull-requests-with-git/
Just execute the following commands and you'll be able to fetch all pull requests by simply executing git fetch pr. Pull requests will be stored in a remote as individual remote branches. So a pull request 123 will be accessible as 'pr/123':
git remote add pr https://github.com/upstream/repo.git
git config --local --unset remote.pr.fetch
git config --local --add remote.pr.fetch "+refs/pull/*/head:refs/remotes/pr/*"