Our experiments show that increased complexity in an essential molecular machine evolved because of simple, high-probability evolutionary processes, without the apparent evolution of novel functions. They point to a plausible mechanism for the evolution of complexity in other multi-paralogue protein complexes.

How "Irreducibly complex" structures form
One way in which "Irreducibly complex" structures could develop is via a process known as scaffolding, wherein a structure gains in complexity via duplication and mutation of parts, then parts are knocked out via mutations, leaving a structure with no direct linear development from the original, basic structure. Other paths by which irreducibly complex structures may form include cooption of parts from other structures. Both of these can be seen in the case of the bacterial flagellum, which includes many parts taken from a secretory pump.

DavidMcC wrote:Natselrox, not every evolutionary biologist has a problem with the term, "irrreducible complexity". The problem is with Behe and other creationists, because they refuse to recognise how it can evolve:
http://rationalwiki.org/wiki/Irreducible_complexity

How "Irreducibly complex" structures form
One way in which "Irreducibly complex" structures could develop is via a process known as scaffolding, wherein a structure gains in complexity via duplication and mutation of parts, then parts are knocked out via mutations, leaving a structure with no direct linear development from the original, basic structure. Other paths by which irreducibly complex structures may form include cooption of parts from other structures. Both of these can be seen in the case of the bacterial flagellum, which includes many parts taken from a secretory pump.

Of course, many people still deride the term itself, because of the reason it was coined by Behe in the first place, in an obviously flawed attempt to show that evolution is impossible. To me, it is just evidence for co-evolution of the parts.