I had the same issue when I upgraded my macbook to osx 10.8. For this reason, I had a look the  the code and  I've found what's wrong .
It seems that OSX's lion noes requires each app to specify the opengl profile at initalisation. For this reason, I have set the view to use opengl's legacy profile (not 3.2 core) in the displayView.mm file. Now Desmune video output is working again in osx 10.8:-)

- (id)initWithCoder:(NSCoder *)aDecoder
{
	self = [super init];
	if (self == nil)
	{
		return self;
	}
	
    NSOpenGLPixelFormatAttribute pixelFormatAttributes[] =
    {
        NSOpenGLPFAOpenGLProfile, NSOpenGLProfileVersionLegacy,
        0
    };
   
    
	dispViewDelegate = nil;
	lastFrameSize = NSMakeSize(GPU_DISPLAY_WIDTH, GPU_DISPLAY_HEIGHT * 2.0);
	glTexPixelFormat = GL_UNSIGNED_SHORT_1_5_5_5_REV;
	glTexRenderStyle = GL_LINEAR;
	
	UInt32 w = GetNearestPositivePOT((UInt32)lastFrameSize.width);
	UInt32 h = GetNearestPositivePOT((UInt32)lastFrameSize.height);
	glTexBack = (GLvoid *)calloc(w * h, sizeof(UInt16));
	glTexBackSize = NSMakeSize(w, h);
    
    NSOpenGLPixelFormat *pixelFormat = [[[NSOpenGLPixelFormat alloc] initWithAttributes:pixelFormatAttributes] autorelease];
    NSOpenGLContext* openGLContext = [[[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil] autorelease];
    [super initWithCoder:aDecoder];
    [self setOpenGLContext:openGLContext];
    [openGLContext makeCurrentContext];
  
   
    return self;
}

http://img571.imageshack.us/img571/4458/desmuneosx108.png

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Can the DeSmenu Mac team members    confirm this "patch" works for them?