GROXX I bought WindowShadeX quite a while ago, and it has the ability to do this to ANY cocoa window. Including the DVD player.
Making things transparent is surprisingly useful, actually. Handy for comparing info, setting up iCal with dozens of events (iCal transparent, info behind, and just draw where you need things).
This, however, is almost totally useless, and unless it does something significantly different than the built-in DVD player, is unbelievably easy to program as well. I don't see how they can charge for this.
(Version 2.1)