Rendering Transparent Text?

Hello everybody,
    I've been scouring Google and I have yet to find anybody who's answered
the question if you can render transparent text with GD. Think of creating a
PNG "stencil" if you will. Take an image that's all black, and then "cut
out" some text on it by rendering some transparent text. I've been trying
all sorts of combinations with imagecolortransparent() to no avail. Any
insight if I'm chasing a pipe-dream would be greatly appreciated.

Thanks,
~Rob