Similar work was done in the OpenTC project (
http://www.opentc.net), i.e., running legacy OSs in isolated compartments and multiplex their visual interfaces using a "secure GUI", which labels the windows/interfaces according to certain properties. It also supported OpenGL in the "AppVMs", which is currently omitted in Qubes OS.
The isolation of drivers in separate VMs and enforced isolation using VT-d is definitely interesting. A paper on the disaggregation of dom0 for improved security is http://www.xen.org/files/xensummit_fall07/22_DerekMurray.pdf