There were supposed to be more cars in Fallout than just the one you got in Fallout 2, but I remember reading that they did it that way because they didn't want the player to just be able to jack one from the locals.
I think cars are appropriate with the 50-style setting; hell, a flying car is the perpetual vision of the future but the status quo seems to be that ground-pounders were still the dominant species at the time of the war. Anyways I'd like to see cars, and different kinds of them, and different things to do with them (Think of the potential for trade that a working car, or especially a truck, brings) just so long as they are never so common that they end up in every single combat as soon as they become available. The effects of combat on vehicles should be harsh -Think Grand Theft Auto; where unless it's armored your car will not survive and extended firefight; but this is the wasteland and you can't just jack another one at your convenience, so in Fallout 3 brining your car into combat should be extremely risky. A climactic battle featuring vehicles is one thing, but their nature as a fragile, expendable, and expensive resource should be clear.
Besides, integrating vehicles into turn-based combat is hellish. Just read over the Shadowrun or D&D rules for vehicles. Hellish I tell you, Hellish. It will render even worse on the PC.
