The Stroke Raster changes the stroke width of a given graphic object according to the gray values of the pixels in a raster image.
This script is now included in the Examples/Rasters folder in the Scriptographer package.
Works in CS4 here. There are two variables:
Grid Size: How big will every pixel be transferred into the graphic object (so 2 is 200%)
Stroke Scale: How thick should the stroke of the graphic object be scaled?
Take into account that the source image will have a line for every pixel, so don't take a high-res image! Try first with something like 100px x 100px or so.