It should be possible to use plain Qt/QML, but I have heard that there are still some issues in the BB10 Qt port. Also it looks like Harmattan Qt Components will not be available. For this reason I'm working on including all the components my applications need directly to the application files (by removinng/reimplementing all C++ plugins so that the resulting components are not dependent on compiled code & libmeegotouch anymore).

The main focus of BB10 seems to be on the Cascades - which is some sort of a QML + Qt Components mashup. If you use it, your application should get all the native look & feel bits. On the other hand it is proprietary & only available on BB10 at the moment. So if you wan't to use Cascades, you will basically need to re/write the UI just for B10.

BTW, I got accepted to the porting program with Mieru, modRana and RePho a few days ago. Haven't got the device yet, probably due to the overall Christmas mayhem.