On the diesel note, Honda is going to be bringing one here as well.
I've talked with several guys who own / have owned the Cummins Dodges, and they get upwards of 20mpg on non-city driving. City, low teens, 12-15 ish.
For comparison, a 4.3L 1500 Silverado will get 20+ highway, not to much lower city. And they cost a whole lot less. Not sure how the S10 does, but comes with the same engine, and is smaller, so mileage would benefit.