"Where? I've never seen an example of such. Since you are stating that they do depict him (or more precisely his face, since that's the actual issue at hand), perhaps you could provide one?"
a google image search for Mohammad turns up quite a few..
here is one on an islamic website:
http://images.google.com/imgres?imgurl=http://www.oranous.org/Image/High%2520Quality/Prophet%2520Mohammad.jpg&imgrefurl=http://aaddee.blogsky.com/&h=1333&w=1000&sz=119&tbnid=Lo8qgYFwTdXLqM:&tbnh=150&tbnw=112&hl=en&start=6&prev=/images%3Fq%3Dmohammad%26svnum%3D10%26hl%3Den%26lr%3D%26c2coff%3D1%26safe%3Doff%26sa%3DG
Well, then, I was right on the rules but wrong on the practices. Interesting. I wonder how many of the protestors and rioters have one of these hanging up in their homes?