I think you can also boot up into your persistent system, add the preferences file to /etc/apt as described above, change your sources.list repositories to edgy rather than hardy, and then run sudo apt-get install [list the upstart files that were pinned]. That should cause the files to downgrade to the edgy version.
Right after that, change your sources.list repositories back to hardy. I am pretty sure I was able to get that to work. I don't recall for sure, but the system may have hung on me after the downgrade. But, when I rebooted, it worked.
Steve's approach may be more of a sure thing (and the approach I describe is subject to the corruption issues that we are attempting to address), but probably worth giving that a try because it is easy to do and might work for you.