I have a from on a tab.
I tried various things:
1. when the frame is on the tab the frame background has the same backcolor as the tab. But the frame's caption is not transparent: the caption has a gray background.
2. The same problem occurs when UseVisualStyle is false.
3. I draw a rectangle behind the frame and set the back color to button face. Now the whole area of the frame is white, although the button face color is gray. The background of the frame's caption is gray.
4. I can reproduce this on XP and Vista.
I don't know if this will be changed. I added some screenshots. Example file to test without rectangle in the background: http://sourceforge.net/tracker/download.php?group_id=187950&atid=923363&file_id=267940&aid=1901612