Version:            (using KDE 4.1.3)
Compiler:          gcc-Version 4.3.2 (Ubuntu 4.3.2-1ubuntu11)  
OS:                Linux
Installed from:    Ubuntu Packages

nvidia card used: nv8400mgs, drivers 177

Following scenarios:

I start nvidia-settings, select a twin view desktop setting. after applying the new settings, nvidia-settings would ask me if i want to keep this settings. that particular window is not showed, instead, kwin crashes and resolution + twin-view-settings go back to what it was before (since the nvidia-settings-timer ran to 0, asking if i want to keep that resolution)
After witnessing this I tried out, in which scenarios kwin crashes, and in which it does not

Kwin crashes only if the twin-viewed desktop is smaller in Y-Size, than the primary screen.
my laptop resolution is 1280x800, my external resolution 1024x768. if i use left-of and right-of as screen settings, kwin crashes. if i use a panning of 1024x800 with nvidia-settings/advanced, kwin does not crash.
If i move the 1024x768 screen by +32 pixels in y direction (absolute: +1280+32 as setting in nvidia-settings) kwin also does not crash! so this seems to have something to do with the taskbar of plasma.

I have to place any desktop "right-of", because the task bar in plasma ignores primary monitor and always displays itself at the most "left" monitor. 
Putting it "below" does not allow my windows to be dragged down to that screen.

My Conclusion
It seems, that KWin and Plasma have issues with some TwinView settings. Especially the behaviour of the taskbar to use always the most left screen, and kwin not allowing windows to be dragged "below the taskbar", and the crashes if resolution is smaller on the extended screen, seem in general like a design problem causing unforseen bugs and crashes.

Note: FYI nvidias Twin-View resizes each desktop by X,Y of the extended screen.