Some of the planned business features would be able to meet you in the middle, but like Joel said, we have no plans to ever sell a self hosted version. We understand that means some companies' policies will prevent some people from using Trello, but its a tradeoff we made.

As an aside, I suspect a few of the companies who, in the past, could "never" use software that wasn't installed on their own systems are probably using Salesforce now.