Qubes-OS

Joanna Rutkowska, the "security on virtualization" guru, started working on Qubes-OS.

In the current implementation, it is mainly an extension to a basic XEN system, which allow you to have e.g. multiple
DomU-level-isolated windows on same desktop.

Unfortunately, currently uses RPM packaging, making it less suitable for deb based distros.
(sorry, I don't trust alien)

IMHO, worth looking.

http://qubes-os.org/Home.html