SJL, you might find this Wikipedia article useful - http://en.wikipedia.org/wiki/Irreducible_complexity

One particular sentence stands out -

Some critics, such as Jerry Coyne (professor of evolutionary biology at the University of Chicago) and Eugenie Scott (a physical anthropologist and executive director of the National Center for Science Education) have argued that the concept of irreducible complexity, and more generally, intelligent design is not falsifiable, and therefore, not scientific.