I don't know about the dynamics of a truck, but diesel cars generally run above the EPA estimates. I don't know if that has to do with the testing or what.
As an example, my sportwagen TDI was rated 30city/42 highway. The car averaged a hand calculated 42mpg combined over the 18 months I had it. During the summer months, i was seeing 46-48 combined per tank, and during the winter 40ish. My mileage is right in the thick of what other owners reported on fuelly as well, so it wasn't some sort of freak.
With trucks, I'd expect they'd do pretty well compared to the EPA estimate unloaded, then fall down as expected when loaded. It'll be interesting to see. If a half ton diesel had been around for a couple years when i bought my current truck, i'd have though long and hard about it, given that I commute 90 miles a day.