I was talking about the 'morris' worm. It had (at least) three methods of propogation, the finger hole, a hole in sendmail and a brute-force login attempt. It attacked two quite different UNIX flavours with different hardware architectures.
I would be the last person to say that a monoculture is a good thing. I was just pointing out the fallacy in believing that a cross-platform worm is impossible. I will go back and state that a monoculture does not increase the risk of successful attack, but it makes the ramifications of such an attack an order of magnitude more severe.