Emissions is the biggest hurdle. You can get diesels in the some states now, but they are either in a VW Rabbit or a 3/4 ton pile driving pickup.
I would like to be able to get a 1/2 ton pickup with a ~4 liter turbo charged 6 cylinder diesel. Put a variable power unit on the dash. Turn it to ~250HP for cruising up to 400-450HP for performance, all without shutting the engine off or leaving the cab. Of course fuel economy varies just as widely. You just can't do that with a gas engine.
Several folks in the general area bought up VW Jettas a bit over a year ago when a VW dealer got several demo cars in. The folks I know are reporting from 42-54mpg. The only problem has been with a few quality control issues with the car itself (not the engine/tranny).