The problem with the diesels, especially the newer clean diesels, is that they are more expensive to produce than gas engines. During our recent dinner with Gary Smyth of GM he indicated that the total coast of a clean diesel powertrain is about twice that of gas. The diesel engines do work though with the new GM diesel estimated to get close to 30mpg on the highway when mated to a conventional six speed automatic. If the new engine were to be matched up to the Two-Mode hybrid system, mileage in the mid-thirties would not be out of the question for the big trucks but at an added cost of several thousand dollars.
[Source: Automotive News - Sub. req'd, thanks to Mike for the tip]