In case you're having trouble sleeping, the final draft of the ECMAScript 4 language is now available at http://www.ecmascript.org/es4/spec/overview.pdf
I'm looking forward to using it, probably much sooner in Firefox than any other browser. Unfortunately, its true potential will never be used until all major browsers not only support the new version properly, but also not until a significant percentage of the population uses the new browsers. By significant, I don't mean "a lot of people", but "enough of the type of people that the bigwigs are interested about"
For the common js tinkerer to play with it like we now play with AJAX, there's still waaaay too many things that need to happen, then. The most important one is being able to say yes to: "can we make more money by implementing this version in our browser?" Otherwise, only a few browsers will have it (yay, Firefox!)
So no, you can't play with it unless you do it at home, after work. Sorry.