Scheduled QJSEngine improvements?
QtScript is not going away for the whole life of Qt5, so no worries. It's also still a maintained module.
V8 indeed proved a bit problematic in some areas, but was ported well enough to be used in QtQuick 2. v4vm is also in the making. I don't know official plans for the API, though, that is a question better suited for Qt development mailing list or IRC.