The big change for Nokia is not the adoption of WP7 as an OS,it is the adoption of .NET as the development platform of choice - not to mention that Nokia's Symbian emulators already used Windows as the development platform. In .NET Nokia is adopting a development platform which provides tools and standards for integrating handsets, slates, and desktops into the back office and cloud while reducing issues of fragmentation. While Qt held that promise, it was a long way from the maturity (10+ years) that .NET provides.

Adoption of .NET will still allow C++ development for people with those skills and inclinations while not adding any more complexity than exists. More importantly .NET will allow more flexibility going forward - even reusing existing VB code or writing new code in F# or IronPython. Qt was not the future for mobile app development because of the relatively high barrier to entry imposed by C++ for casual app developers and enterprise IT departments.