That's not being addressed in regards to these updates in Qt. To my knowledge (and I might be fully wrong here) but Qt 4.7 will not be on the Nokia N810. Qt Mobility will never make it to Maemo 4.1 (Diablo) either. What does Qt offer in those situations once a device is old enough to not be supported by the new features? It's all fine and dandy at this moment. Not so in the future. I think that part needs to be confirmed about how forward reaching these updates will be.
In the case of Diablo, it might not be that much further.