I saw an incredible presentation on this at the MIT monthly Lisp meeting by the ex-OLPC security guy and BitFrost architect Ivan Krstic. Fascinating idea, and something that made me really want to explore the uses of JavaScript in terms of security.
It also made me wonder about the feasibility of implementing a capabilities system in CLisp or Scheme. So many ideas, so little time...
The spec is great reading, too [PDF]: http://google-caja.googlecode.com/files/caja-spec-2008-06-07...