Joanna Rutkowska has demonstrated the importance for some very low level controls in modern operating systems (and virtual machines).
'pdp' at GNUCitizen has a recent write-up of browser rootkits that is good-- it talks generally about the problem, not specifically. I would like to see him propose potential solutions to his concerns, though, which he has not yet done:
"As you can see, browser rootkits are probably the future of malware. In the wrong hands, browser technologies are power tool that can be used to keep unaware puppets on a string. I am planning to follow up this post with a more detailed example of how browser rootkits are developed and show some interesting functionalities which can enable attackers to go so deep, no other has ever been."UPDATE: Joanna Rutkowska chimes in.