While it's not running INSIDE the browser per-say there is http://www.browserling.com/ that'll let you use another browser inside another browser (and use browserling inside that other browser if you need to go deeper).
With all due respect to the fine folks at Opera Software, I think they should have much bigger concerns than the possibility that Chrome OS will ever hold anything close to a monopoly.
What will be interesting is to see how long we have to wait afterwards until someone decides that it might be a useful thing to do in production.