>>tete+(OP)
While I am all for virtualizing, it doesn't help security. It just moves the exploit from your OS into your hypervisor. Even worse, you add a whole new level of exploitable code.
>>mrotte+Bl
Of course it improves security. On Qubes, someone who can exploit your browser (pdf reader, word processor) doesn't automatically get free rein on your machine. They still need to escape Xen.