A new truck will depreciate fairly quickly (not as quickly as a car, but still at a good clip). An older truck, in good shape, can hold it's value for long periods of time, especially if it's driven less than average and stays low mileage.
I paid $15k for my truck a few years ago, and while I do intend to keep it the rest of my life, I could easily turn around and sell it for nearly what I paid, if not more (there are a few grand of upgrades I've put in as I repair things). It's a low mileage (just rolled over 100k miles) '97 F350 with the 7.3 Powerstroke, and it's in very good shape, cosmetically and mechanically. I don't put that many miles on it, as it's not my daily driver (working from home helps a lot there). It'll rack up 2000-3000 miles/yr, mostly in longer trips, and I keep it well maintained. At 3000 miles/yr (which is probably about right, even if we're doing some cross country trips), another 200k miles will be 66 years, which should pretty much cover me for life. :) And I'm in a dry climate, so rust isn't a problem.