Serge Hallyn of IBM and general kernel hacking fame has written a great article on Role-based access control (RBAC) in SELinux.
The article is also something of a tutorial, implementing a security scheme for a simple store cash register system, with downloads available for a Gentoo-based SELinux from Scratch qemu image; and for standard Fedora 8 systems.
It’s great to see these kinds of projects coming from the community!