Because this is buried deep in the release notes, I have a feeling we’re going to see a bunch of people Googling this question, so I figured I’d get it out of the way
The short answer: No.
“Google Native Client support is not functional in Unity 4.3. If you need to publish to Native Client you can still use Unity 4.2.2.”
The longer answer:
In a nutshell, the Unity devs don’t feel that Native Client has lived up to the hype. They were hoping for features that haven’t materialized, and it doesn’t appear to be getting a lot of love from Google ATM. It appears that Unity just doesn’t want to expend the extra effort to support it as a platform, along with the other dozen. I’m guessing they view it as a distraction vs pumping out new cross-platform features, which they would rather do.
However, browser plugins are going away in a big way. Google, for instance, has announced that they are totally abandoning NPAPI, which many common plugins are built on, as early as next year:
Mozilla has added some security restrictions for plugins so they don’t autoplay. IE may restrict or remove plugin capability as well. In that case, we may see a bigger push for technologies like NaCl, because as of right now there aren’t a ton of good alternatives.