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 a wonderful world, on one end you have people moving client side code to the server (node.js, Dart, Meteor) on the other hand you have people trying to move it to the client (GWT, Doppio)