As mentioned it seems to be a problem related to Desktop Composition.

That feature of aero is what lets the translucency work. Each window is rendered into it's own bitmap with Alpha channels. Desktop composition is what does all of the layering of one bitmap on the other.

The 6150 seems to operate the same with desktop comp on or off. It's just that as it shifts into Secure Desktop, there is a 5-6 sec blank screen delay, just before the dimming effect takes place.