Sorry to sound like a negative Nancy but how will this prevent fragmentation of JS? I read the weblog, the ecmascript link and the Harmony requirements but nowhere does it say how to tell whether the runtime supports the JS/Next script or which of the big names has commited to following it.
Members of the TC39 committee include representatives from Apple, Google, Mozilla, and Microsoft. Javascript engines from these vendors will almost surely implement the standard. Firefox has prototypes for a few new things already.