Just to follow up on my last post. The only 1/2-ton diesel project I'm aware of that hasn't been canceled is by Nissan, which is looking at a 2.8-liter 4-cylinder Cummins. The goal is to get an EPA rating of 28 mpg (combined):
Cummins developing 28-mpg 2.8-liter 4-cylinder diesel for Nissan Titan — Autoblog Green