I can't speak to the overall progress (they've implemented a bunch of stuff I know there are big missing features). As for `yield`, I spoke with the developers a few months ago and they said that it's one of the next big features they'll work on. Don't expect it soon though, I bet it will take a while.
Considering that ES6 has pretty much been approved, and is expected to be finalized in about a year, we will definitely see this in most browsers in a year or two. It's a little ways off, but it's definitely happening.
Strictly speaking JavaScript 1.7 is Mozilla's own programming language. V8 does not implement it, instead it implements ECMAScript as described in the ECMA-262 standard. Latest edition of ECMA-262 is 5th and it is fully supported by V8.
Now some features from the 6th edition (that is currently being worked on) are somewhat supported by V8 but they are all hidden behind the --harmony flag.
Generators are actually also in ES6 (though they are a bit different from JavaScript 1.7) so sooner or later they will make their way into V8.
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