Github have added a 'revert' button next to pull requests: https://github.com/blog/1857-introducing-the-revert-button
When using Bitbucket I still need to use the manual steps documented here: http://tildedave.com/2012/11/24/reverting-a-github-pull-request.html
In summary I have to find the merge commit and then run the command line:
git revert -m 1 <SHA-1>
Please add an equivalent to Github's 'Revert pull request' button into Bitbucket.
This would be very useful for continuous integration / deployment workflows, especially as I can't revert a merge commit in sourcetree - See related question in Atlassian Answers: https://answers.atlassian.com/questions/322639/how-can-i-revert-a-merge-commit-in-sourcetree.