I've thought a bit about this. In addition to putting rollcages in all of my off-road vehicles, I'm the type of person that would put a rollcage in a sedan, minivan, or just about anything else on four wheels. Think about the fantastic crashes we've seen racecar drivers walk away from, and then consider how many people die in relatively low-speed traffic accidents. Most of the cars on the road aren't nearly as safe as they could be. But building a race-quality rollcage is EXPENSIVE ($3-5,000 minimum) and most people don't want to deal with the inconvenience. Personally, I don't want to go out like that.
This is my daily-driver. I built that rollcage to survive cartwheeling through the desert at 80mph+, so I feel pretty safe in that truck on the road.
I feel pretty safe in Mah Deuce too though. Like Ken and wreckerman have already pointed out, we're not too likely to roll these vehicles if we drive them responsibly. But it's the blowout, or the brake failure, or some other catastrophic mechanical failure that I worry about most. I'm also concerned about the real possibility of a low-speed flop off-road. I drive an M109 and I can see myself getting my truck in a situation where that's a possibility. I think the height of the box on my truck will protect me in most cases, but well, I have all the tools and the ability to build a nice rollcage.
So I'm planning to build an four-point, in-cab rollcage for Mah Deuce. I'll probably use 2" x .135-wall DOM tubing (because that's the largest die-set that I have/can get for my bender). I'm going to run tubes along the floor, around the perimeter of the cab. This will serve as the foundation for the rollcage, because I don't want to tie the rollcage into the frame (because the cab is not rigidly mounted to the frame). The seats will be mounted to this tubework on the floor, and the whole point is to anchor the rollcage to the cab in such a way that it will not just punch through the sheetmetal on impact. Then I'll build a simple hoop that follows the windshield frame, and another hoop at the B-pillar (in the rear corners of the cab, behind the driver/pass). I'll triangulate that B-pillar hoop somehow, and same thing goes for the overhead bars that connect the front/back hoops. There is plenty of room in these trucks to build something like what I'm talking about in such a way as to not be intrusive to the driver/passengers.
Whenever I get around to designing/building this, I'll post blueprints (SolidWorks & BendTech Pro) to share with everyone. But this is really a pretty simple project. If you don't have the tools and the know-how, any competent fab shop should be able to build a nice cage like this for $1500+-.
Here's the thing though: I wouldn't expect a rollcage as simple as what I'm proposing to offer much in the way of real protection in an a high-speed roll, and I wouldn't be surprised to see it to get seriously deformed/damaged in a low-speed/off-road flop. Little more peace-of-mind than a hard/soft top though...
Installing a headache rack or Smittybuilt-type bed bar isn't going to cut it. BUT, I think that building a heavy-****ing-duty version of something similar specifically for our trucks would be a really good idea. I can see something like that doing even better in a crash than the rollcage I'm planning on building, because it could be triangulated in ways that wouldn't be practical inside the cab of these trucks. It wouldn't keep the cab from getting crushed, but if it holds up, it might keep the cab from flattening at the rear (you know, where our heads are). Tubing-wise, I'd go big. Find a shop that can bend 3"+ tubing. You'd only need four bends in one piece of tubing. Shouldn't be too expensive.
If anyone wants to build a rollcage/rollbar for their truck, I'll help with the design.*
*As long as you understand that I'm just a designer/fabricator, not an engineer. I can't/won't make any claims as to the safety of anything being discussed here.