Well, I didn't imagine it, otherwise Erik wouldn't have marked it as open.
I realise you've made broad changes to pull request functionality, so this bug may no longer be valid for that reason. However, I notice that a number of pull requests I had made some time ago have disappeared apparently without trace since your changes:
I sent a pull request to izi/pilib on May 28, 2011.
I sent a pull request to mchaput/whoosh on June 9 2011. I believe this was fulfilled.
I sent a pull request to ianb/pip on 1 March 2011.
For all of the above, there's no record of them that I can see (either under Open, Fulfilled or Rejected). Have you just deleted all the old pull requests? Have they been deleted by the recipients? Shouldn't a pull request originator be notified? When closing a request which was valid because it's been superseded by other developments, a comment to that effect should be added.
The old pull request system really just sent a pre-composed message to the upstream repository owner. Migrating those to the new system presents a number of challenges:
We'd need to parse every message in every user's inbox and look for ones that look like pull requests. This probably wouldn't be too bad, since all of these messages should have a subject of "Pull request" and end with "Repository is at <repo url>".
Pull requests created from this migration would have no information about what changes went out or how they were merged. You'd really only get to see a description.
New-style pull requests don't yet support comments, so we wouldn't be able to migrate any replies from old-style pull request messages. We will eventually have comments for pull requests, however.
The old-style pull requests and their discussion threads have, for now, been private. If anything private was discussed in those threads, they'd now potentially be exposed to a wider audience on the site.
I think once we've added comments to pull requests we'll be in a better place to think about making this sort of migration, but unfortunately it's a pretty complicated situation.
All of that said, you can still see those pull requests in your inbox or outbox on the notifications section of the site.