First, start out with a plain white background. Now, add Noise (Filter/Noise/Add Noise) with a setting of about 300. Now, create a sort of stained glass texture by crystallizing it (Filter/Pixelate/Crystallize) with a size of 10 or so. Now, round the blocks with the Median filter (Filter/Noise/Median) with a setting of 6. Finally, apply the KPT filter, Find Edges Soft (Filter/KPT 2.0 Filters/ Find Edges Soft) If you don't have it, you can download all of the KPT demo filters here (Sorry, no Mac yet) Or, if you don't want to download the filters, you can apply the Find Edges filter that comes with Photoshop (Filter/Stylize/Find Edges), invert, and blur slightly.