Unfortunately, this
concept neglects the importance of
reuse in evolution. One of Behe's examples is the bacterial
flagellum that is a
complicated machine that would not work without all of its parts. This line of reasoning ('it cannot work as anything less than 100% complete') is used to argue the impossibility of stepwise construction. Also known as the 'what use is half an eye' argument.
The simple solution to the question is that the parts were used for something else. That is to say, they once had a different function (perhaps an ion pump for the flagellum). This function was overridden by the organism to provide a new structure. This scavenging tendency is the reason for the homology found between genes in different branches of life. It is also why moonlighting proteins are relatively common (such as the crystallins).
Unfortunately the argument is based on the usual tyranny of probability. For example, consider a two component system : S = {C1, C2}. The principle of irreducible complexity says that the probability of the system evolving is:
P(S) = P(C1) * P(C2)
In words, this is (probability of the system evolving) equals (probability of component 1 evolving) AND (probability of component 2 evolving). If the components are improbable themselves, then the product of their probabilities is very small. Indeed, since proteins are often described using
infinite monkeys, the product of several small numbers is very small indeed.
This is, of course, rubbish. It ignores the simple fact that systems evolve as a whole. Ironically, it is the very tendancy to assume things are instantaneously created that leads people to such assumptions. Nobody seriously thinks that systems like this jumped into existance without any predecessors. More importantly, the ancestor systems were fully functional.
A key point for Behe to think about : evolution can be irreversible. Just because a system seems unstable to change (irreducible, in other words) does not mean it couldn't be evolved. In other words, consider a sequence of systems (or {S1, S2...Sn} ) which increase in irreducible complexity over time. That is, they come to depend more and more on each other through interlocking changes in their components (C1 and C2).
Clear? :)