No one knows how to do this!!

[korbbit]

ive looked all over the net, this technique has to be the most well kept secret ever.
there is a very simple effect im trying to acheive.
i have scanned a greyscale pencil drawing and set it as a layer with "multiply" blending mode, so i can colour benath it. the problem is now i have to get this out of photoshop as a png or somesort of transpanercy supporting file without the white background.
how can you acheive the effect of applying(or making permanent) the multiply blending mode? so white pixels are 100% transparent, black are are 0% transparent, and a grey pixel would be black and 50% transparent?

[SCTRWD]

Simple old technique: Return mode back to Normal, select the RGB channel (Ctrl-Alt-2) and press delete key.

[photoshopgeek]

I think it would also be possible to do it through the layer styles. In the blending options at the bottom it has the "blend if" sliders. This may give you more control.

Haven't used them much though, so I may be wrong.