Oliver Morton at Nature pointed me to a bunch of excellent posts on Coskata by Robert Rapier at R-Squared.  Recall that Coskata wants to gasify cellulose and feed the resulting synthesis gas to bugs that make ethanol.  Here are Rapier's "Coskata"-tagged posts.

Among other points, Rapier makes some nice back of the envelope estimates of the technical and economic feasibility of Coskata's process.  In short, Coskata's claims appear to be consistent with the laws of thermodynamics, but perhaps not so much with the law of supply and demand, and their logistics challenges might border on being inconsistent with the consevation of matter.

Basically, it all, err, "boils down" to the fact that Coskata is probably going to get tripped up by their focus on ethanol and the consequent energy cost of separating ethanol from water.  Even if you have a nifty process for turning cellulose into ethanol, it takes a large fraction of the energy in the cellulose to purify the ethanol.  And it really doesn't matter whether you distill or use a membrane -- the entropy of mixing still hoses you even if you somehow escape the specific heat of water and its enthalpy of vaporization.

Now if you hacked the metabolic pathway that consumes synthesis gas so that the bug made something more interesting like butanol, or a gasoline analog, that either had lower miscibility or even phase separated, that would really be something because it would minimize the energy cost of purification.

Great work, Mr. Rapier.  And many thanks, Oliver.