At the IATF conference (see below) a member of the Secure Trusted Operating System Consortium spoke with myself and Keith Jones. This group is trying to build a "trusted" operating system using the underlying Apple Darwin operating system.
Being a BSD fan, I should give the OpenDarwin OS a try. The main obstacle appears to be limited hardware support, although I expect that to improve. Thankfully, on the software side their is a Darwin Ports project to keep the great BSD ports system working for this Apple project. The list of software is fairly small right now though.