Fog Creek: A version we can host ourselves, please! We would pay fogbugz-levels if not more.
It wouldn't even have to be updated anything like as frequently as real Trello, or at all. A stable snapshot, with security updates if you found egregious holes, would be more than enough.
"When you said it had some rough edges, I thought you meant ..."
I'm only 90% kidding.
I work with Agile teams, and putting data on other people's servers is a BIG hang-up for a lot of companies. Even when we assure them that it's not requirements or specs, just symbolic work tokens, they still freak out.
If you encrypted it client-side, you could make the argument that nobody without the encryption plug-in could see any of it -- that you're just storing encrypted blobs somewhere.
It probably still wouldn't fly -- I have my suspicions that much of these conversations revolve around control, not security or policy, but it would be worth giving it a shot.