I've now fixed the bug and want to resubmit the pull request with 1 extra commit. Is there any way to reopen the pull request or update it, or do I have to create a new pull request, type out the description etc again? Gitorious has this feature and we've recently moved to GitHub.
The answer seems to be: You can't.
Once a pull request is merged and closed, it is locked forever and cannot be reopened. If your pull request is merged, closed, then your changes are pulled out (via force pushing backwards to before the merge), you will need to add commits to the branch and create a new pull request, copying all the details over and probably providing a link to the original pull request to manually save the history.
Might be a nice feature request for future GitHub.