It isn't supported yet, but should be in the PlayBook 2.0 OS.
We will have to see how well it works, and what is supported.
The promise of "cross platform development" has been greatly exaggerated up to this point for mobile applications.
Don't get me wrong, it was a lot worse, but a Qt application isn't a compile away for most situations.

For example you can also use Qt in an Android application using necessitas, but currently it lacks a number of modules (multimedia, sound etc.). And when Qt for Symbian^3 was released OpenGL wasn't supported yet. You still can't compile for Symbian on a non-windows platform using the official SDK. WebOS also used to have some Qt components, but without proper integration with (a version of) Qt Creator that isn't very useful.

So I really hope the Qt support functions well in PlayBook OS 2.0, but that is something we will have to wait and see.
If it is properly integrated with Qt Creator, can be compiled on Linux, has everything needed to make a game (at least partial OpenGL support, and sound/multimedia), then I will buy a PlayBook to test it. Even if I think that the tablet is really unoriginal in both hardware and software design.