In this thread, Robert Purves wrote:

>In my tests, the suggestion has no effect on the black result from
>CGContextClearRect.
>
>On the other hand, setting kWindowIsOpaque causes a document window
>to be drawn with an interesting cosmetic defect at the rounded top
>corners.


Robert,

My fault for not testing the suggestion, but here is the originating thread:


http://lists.apple.com/archives/carbon-development/2003/Jul/msg01225.html


You can download an example file of kWindowIsOpaque in action here 
(no need to compile the code, just download the tiny DMG and laucnh 
the app in the build folder) to see the funky result (which appears 
broken here under Panther with an extraneous button, but may work 
better under Tiger):


http://developer.apple.com/samplecode/CarbonTransparentWindow/listing3.html



You and Bernie might find it of interest.

Ken