You can also do that by just creating an Issue, and then converting it to a Pull Request later with `hub pull-request -i <existing-issue-num>`, when you actually have some code to commit. That uses the software Hub, mentioned in the cheat sheet. Or instead of installing Hub, you can convert the Issue using one of the other methods listed in http://stackoverflow.com/q/4528869/578288.
I use this constantly. Building a PR up with lot of people watching and involved is pretty amazing. Certainly don't need to all the time, but when working on something sensitive or still ambiguous it's invaluable. So many of my PRs have 50-70 comments by the time they go in.