This is and old patch (June, 2009). The reason it never made into trunk it's that we (me at least) thought that
it should be optional, as a ui or command-line option.
However, it seems that OSX always have the OpenGL context with alpha enabled. So I think it's consistent to
have the other OSs to follow.
The main usage of this is the BGE (or more specifically people using BGE for TV broadcasting) but I think
pydevs can have their share of fun with it in Blender as well.