Random monotone function
I'm not sure, but I think that the problem here is the fact that we don't have any strong assumptions about pseudorandom monotone function generators (at least none that I know of). The idea of the proof in Razborov-Rudich paper is as follows: 


  if there is a natural property of functions (i.e. an efficiently
  decidable property that holds for
  sufficiently large subset of functions
  and implies that the function needs
  big circuits), then it can be used to
  break pseudorandom function generators
  (which breaks also pseudorandom
  generators and one-way functions).


If we were to restate the theorem in terms of monotone functions and monotone circuits, we would like it to say 


  if there is a natural property of monotone functions (i.e. an efficiently decidable property that
  holds for sufficiently large subset of
  monotone functions and implies that
  the function needs big monotone
  circuits), then it can be used to
  break pseudorandom function generators
  (which breaks also pseudorandom
  generators and one-way functions),


but now the proof from the paper stops to work, because our pseudorandom generator outputs general functions, not necessarily monotone ones, and we cannot use our natural property to break it, because even a relatively large subset of monotone functions will not be large relative to general functions, for the set of monotone functions itself isn't large relative to the set of all functions ( http://en.wikipedia.org/wiki/Dedekind_number ). We could define some pseudorandom monotone function generator and use the natural property to break it, but we probably would not have the equivalence between this generator and one-way functions, so the theorem would not be so interesting.  

Maybe this difficulty can be fixed (but I don't think it follows from the proof in the paper in a straightforward way) and maybe the problem with the monotone functions lies somewhere else. I would really like someone more experienced than me to confirm my answer or to show where I am wrong.