Slowmover's metric is right on. While there are certainly political and social reasons to use less petroleum, isn't our desire to spend less $$$ to get somewhere else the relevant point?
I haven't run the numbers on trucks because I wouldn't want one no matter what, but diesel SUVs, even with their higher mpg ratings, don't seem to have better cents-per-mile capability than their gas engined siblings. Diesel fuel being 20% more expensive than gasoline mostly negates their efficiency advantage. When you combine that with higher maintenance costs and higher purchase price I don't get the attraction.
I was very interested in a diesel jeep GC but just couldn't make the cents-per-mile work out better than the gas engine model. I'm glad I bought the V6 every time I see the gas station sign with regular at $3.30, diesel at $4.00.