This V8 engine isn't living in the mainline Qt tree but in a separate Git repository for now. "The status of the V8-based back-end is that we still have some QtScript API that’s not implemented, and there are autotests failing (QTBUG-17640), but several of the QtScript and QML examples and demos run. Aside from behavorial compatibility with current Qt, we also need to ensure that there aren’t any performance gaps in the C++/JS layer before a V8-powered Qt can become a viable solution. As I mentioned in a previous post, we’re simultaneously looking to trim the fat of our current APIs to make a switch feasible."
The Nokia post was by Kent Hansen and in a follow-up comment, he clarifies that this is currently a research project. "It's not a decision, it’s a research project. V8 is an interesting technology to work with. The background is that someone ported QtWebKit to use V8 and got some encouraging benchmark results, so we wanted to explore this option further. We’re not out to fragment anyone’s ecosystem here."