i think Qt is a problem for Nokia now. It costs money and with the choice for Windows and S40 they probably can't use it themselves in the future.
But because Qt is open it does enable competitors to use it. RIM already stated that it wants to use it, it was used by WebOS, there are (incomplete) ports for iOS and Android and maybe Ubuntu also plans to use it in their mobile phone plans for 2014.

So Nokia may actually try to really kill it as best it can. Since they own Trolltech, they could just decide to stop any business activity it has and fire as much people as they can. That could delay any future development very effectively, even if it is open source.
If they sold it, it wouldn't cost as much, but they would directly help a competitor with their years of investment.