Juriy Zaytsev (@kangax) just today released a really good article on the subject.
He explores various alternatives like the Dean Edwards iframe borrowing technique, direct object extension, prototype extension and the usage of ECMAScript 5 accessor properties.
At the end there is no perfect implementation, each one has its own benefits and drawbacks.
Definitely a really good read:
- How ECMAScript 5 still does not allow to subclass an array