Hi originquestor.
A couple of comments on your post. In the first place, Behe (not Dembski) is the originator of the idea of irreducible complexity inre flagella. Dembski's the guy who developed the concept of specified complexity and the explanatory filter that is supposed to identify true specified complexity from apparent specified complexity in nature (based on probability calculations). Obviously Behe's and Dembski's concepts are related, and Dembski is now spending an inordinate number of words trying to rescue IC from the dustbin. I think he's making a mistake, 'cause by linking the two ideas so closely, if one is falsified it automatically falsifies the other.
As to Behe's flagella in particular, he's referring to eubacterial flagella. This is a key thing to remember, as the other two basic types have already had a great deal of research behind them - showing how they are not irreducible. One problem that Behe mentions (as you did in your post), is the existence of the so-called "o" rings (basal rings on the rotor). At first glance, since the only known homologues are Type III transport systems, and the fact that there are no known "primitive" versions (afaik), it looks like Behe might be on to something. However, by definition, if something is "irreducible" it means there is no possibility of variation - it either IS reducible, or it isn't. In the case of the basal rings, there are a number of eubacteria that have different configurations. For example,
E. coli has four rings,
Bacillus subtilis two rings, and
Caulobacter crescentus five rings. As I have said before on this forum, it's fairly easy to imagine a scenario where a "primitive bacteria" might have one ring, and then you have a flagellum with two rings, then three, and so on through the well-known processes of gene duplication, fusion, etc. In other words, Behe is taking a fully-evolved "modern" organism with 4 billion years of evolution behind it, and saying (basically), "I don't know therefore god/designer didit". Looking at an organism
post facto and declaring that it "couldn't" have evolved is quite different from looking at the organism and asking, "how?".