I asked myself what branch rebase refers to. The one I want to rebase on or the one I want to rebase from?
Also notice that, like the git merge command, git rebase requires you to be on the branch that you want to move.
Thanks to the great Rebasing article I know now :). (And store it here, to know where to look it up).
In short. If I want to get my branch about to be back on top of master, I have to rebase it onto master, like this:
git checkout about
git rebase master
As with everything, behave when rebasing! I believe that rebase is not meant for all situations. Sometimes it is much better to have a history visible in the commit history.