Toolbar background not transparent

There seems to be a problem with the toolbar background on the final release of Vista. The screenshot shows an icon on the left which is just a black to white gradient. As you can see, no variant of gray matches the background colour of the toolbar, it's as though TLB is rendering additional transparency. I've tried changing every setting I can think of, but nothing gets rid of the problem. The exact same setup with the same version of TLB and the same icons works fine on XP SP2.