At first, Qt 4.2 is still at development stage. Maybe this has not yet been fully implemented. Are you using the tech preview or the latest snapshot anyway?

I have the feeling that the corner widget is intended to be something smaller than another scroll area. If I got it right, the "corner widget" will appear in the bottom right corner (in LTR layout) between the scroll bars. How does a widget with yet another scroll bars fit into that tiny area in between the scroll bars?

And notice what docs have to say about this:
You will probably also want to set at least one of the scroll bar modes to AlwaysOn.
This will assure that the corner widget has some space to show itself.