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.
It's not that we have inflexible arbitrary rules, but rather, considered reasoning as to why we don't want to depend on externally hosted services, and sufficient competitors in most spaces that are willing to provide self-hosted products. Companies such as Atlassian even go so far as to provide the source code, as do others, which has allowed us to maintain critical (non-atlassian) services well past the time they were discontinued by their parent company.
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.