"ActionScript 3" is more than "slightly" different:
http://livedocs.macromedia.com/specs/actionscript/3/wwhelp/wwhimpl/js/html/wwhelp.htm

I still believe that it is slightly different than ECMAScript Level 4 (JavaScript 2.0)

Yes, I think some developers from Abobe indicated that ECMAScript 4 will be fairly close to the current AS3 language. The new cooperation between Mozilla and Adobe increases the chances of that happening.

However, as far as I know there are no published language drafts at the moment, so there is no guarantee.

In addition, implementing an meaningful alternative to the Flex tool might require working with some technologies that are not open/published. My knowledge on this is limited, but I think that binary .swc files (analog of Java JARs) containing compiled code are not specified. (Although there are many tools available these days)

I saw
http://www.mozilla.org/js/language/js20/index.html and http://www.mozilla.org/js/language/es4/index.html
Besides Wikipedia states that fact (and my eyes :)

I have not followed the progress of ECMA TC-39/TG-1 for some time, but I think those pages might be quite old.

It may be a good idea to contact Brendan Eich: he might be able to give relevant status/information.
http://www.mozilla.org/projects/tamarin/

Looks like the Actionscript 3 is a subset of the eventual ECMAScript 4. I don't see the "let" keyword or any of the funky array syntax in AS3.

I assume this will be implemented as a separate language, parallel to the current JavaScript language, correct?

I believe for now that it will be separate JS dialect to reuse as much existing js code as possible

Most up to date spec http://developer.mozilla.org/es4/spec/spec.html