I don't know about diesels:
a. Gas engines with direct injection, turbos, etc, have closed the efficiency gap significantly.
b. Europe is banning diesels (Paris to be diesel free by 2020; London is following).
c. Current gas prices are down $1 USD/gallon since last year (Diesel hasn't come down as much ($1USD/gal more in parts of NE, and up here in Canada). Means a diesel will never pay for the higher initial cost.*
*Note my crystal ball isn't clear on when gas prices will go back up. But, I am betting Chevy delays the diesel in the Colorado/Canyon (or puts just enough out for marketing hype - pickup with best fuel economy; taking title from Ram).