Summary: The new 'sandbox' feature on <iframe> should be considered for removal. It needs a security review, it will be a lot of work to implement properly, and may not actually solve the problem it is intending to solve.
More details here: http://wiki.whatwg.org/wiki/Iframe_Sandbox I encourage fellow web authors and browser implementers to add their opinions/comments to that wiki page. Thanks! Tantek -- http://tantek.com/ - I made an HTML5 tutorial! http://tantek.com/html5