Transparent sheets in Snow Leopard

Hi all,

I've written an application where I have a transparent sheet showing  
over a window. I use this to show text (or other things as well) over  
either a background image or a video.

To make the background of the sheet transparent I sublcassed NSWindow  
and used the following:

/ 
*******************************************************************************
  * Function:    initWithContentRect
  *
  * Returns:     id of transparent window
  *
  * Description: Sets up the values to make the window transparent
  *
   
******************************************************************************/
- (id) initWithContentRect : (NSRect) contentRect
				 styleMask : (unsigned int) aStyle
				   backing : (NSBackingStoreType) bufferingType
					 defer : (BOOL) flag
{
     NSWindow* result = [super initWithContentRect : contentRect
										styleMask : NSBorderlessWindowMask
										  backing : NSBackingStoreBuffered
											defer : NO];
     [result setBackgroundColor: [NSColor clearColor]];
     [result setLevel: NSStatusWindowLevel];
     [result setAlphaValue:1.0];
     [result setOpaque:NO];
     [result setHasShadow: NO];
     return result;
}

This has always worked for me, but now when the sheet is showing, the  
area covered by the transparent sheet is blurred. I've uploaded a  
screenshot to http://www.jacobs.co.za/BlurredSheet.png

I've recently upgraded to snow leopard and I don't remember this  
happening in tiger or leopard, but that may just be a coincidence.

Can anyone help me out? Thanks in advance!

Trent.