OK, it's amazing how far off-topic this thread got

Rendering graphics (like the dock) over an overlay will inevitably black out that region, due to the way overlays are handled. I think you can get around this behavior if using VMR9 instead of overlays, but there isn't a lot of VMR9 support, and the fix has to be done in the app playing video, not the dock.
and fwiw: apple has much greater control over hardware than ms, so its easy for them to write an api like quartz or core, and then say that anybody with a computer more than 2 or 3 years old has to upgrade to run the latest version of osx. ms is much more concerned with backwards compatibility than technological advance (for better or worse) which is probably why they work on api's like gdi+ that don't require 3d hardware, pushing stuff like avalon out into the far future. a look at their history: wimp interface, internet explorer, msn messenger, 64-bit, etc... has always shown that they care less about being *first* than being #1 in market share.