If you read almost any science blog other than mine, you're probably aware of Brown University biologist Ken Miller's smackdown of Intelligent Design (ID) shill Casey Luskin, posted on Carl Zimmer's Loom: part 1, part 2, and part 3.

At issue is the tired old concept of irreducible complexity, and it's amazing that after all this time, many ID advocates don't understand what the original point of arguing irreducible complexity was. ID advocate Michael Behe, in various publications including his book Darwin's Black Box basically argued that there are molecular systems inside of cells that, even in principle could not have been produced by evolution - systems like the bacterial flagellum and the blood clotting cascade. Such systems, according to Behe, are irreducibly complex - they need all of their parts in order to function, and if you're missing any parts, you have a non-functional system. Thus, without all of the parts there is nothing functional for natural selection to act on.

In other words, the only way evolution could produce a system like the blood clotting cascade would be to have all of the relevant genes suddenly appear at one time by mutation - an event improbable to the point of impossibility (which is one thing ID advocates and evolutionary biologists agree on).