"*You can generate a quick and dirty transparency map by inverting the colors of the texture map and grey scaling them. There is actually a way, more or less, to do this in the material room, but I can't quite remember it. You can then blend this transparency map with some appropriate noise.*"

Subtract original from white = "negative" colours.
Plug into a math node to convert to greyscale. (Actually, not needed when used for transparency or any other node that requires grey-scale; the conversion is done automagically).

Hope that helps.

Cheers,
Diolma