I think in the spirit of keeping it real, a diesel Subaru is going to get at best about 30-35MPG highway unless they drop AWD.... There's no way, given the weight that Subarus carry around that they're going to be posting highway mileage in the 40s purely by using any diesel engine that would provide enough torque and horsepower for American demands.
Don't get me wrong, though, I do like Subarus... there's just a reality of AWD and heavy cars like that....
I hope we see more clean diesel launches on the car side, though. Clearly the US automakers have been selling diesel in trucks for years, and they're committed to clean diesel in truck engines. But I think it makes a lot of sense in cars, and I want to see more of it. Maybe even some kind of diesel hybrid electric design -- for instance for Subaru, a diesel FWD + electric motor rear-wheel assist might work out really well.