The Photoshop 'Dissolve' blend mode reduces a layer to random dots in a density based on that layer's opacity. If the layer opacity is controlled by a grayscale image layer mask, then a simple stochastic halftone can be produced.
More on halftones in Photoshop (using the 'Hard Mix' blend mode)
More on using the 'Dissolve' blend mode (for additive color mixing)
UPDATE 6/29/2009
For comparison here is a real stochastic error diffusion FM frequency modulated halftone screen made by putting together three Photoshop bitmaps.
Not a comment, but just a thank for all of this!
amicalement,
SG
Thank you :)
Terrific!