When you say dark - is it the same shade as the background when there's a dialogue box in the foreground?
My experience is that your problem occurs much less with the current testing version (21.2011.38-1Tmaemo1.1). It is usually caused by dialogue boxes maintaining focus but being out of screen space.
To prevent the problem, try locking the screen to landscape.
- Does it still happen?
- How much?
- What else were you doing (e.g. rotating, quitting a phone call, etc.)?
- Does it only happen with the phone application? (IIf not, what else)

I remember correctly, the original Maemo5 UI rules required the "go to window-manager" button to be visible at all times but I think this changed with the roll-out of Qt. The way Maemo5 is headed, it's going to work just as well in portrait as it does in landscape with the kb out; there is no other OS capable of doing this, so it will take a few iterations to get there. It is likely that the problem you/we are experiencing is caused by a combination of the CSSU and other software, as opposed to the CSSU alone.

There seems to be no means of "rescuing" a programme that has produced one of these errant dialogue boxes. Is this correct? If so, how can it be resolved?