I put one of these things on my Toyota Taco when I moved out to California recently:
Catalytic Converter Theft Prevention Lock
It's a bit pricey (~$180), but cheaper than a new cat. It installs easy. The emissions guy out here in Cali looked at me like I was an idiot, but it is some piece of mind.
I think it will work as a deterrent when the thief sees it, he'll probably just move on to another truck. If not, the cables are thick and would be really tough to cut with anything a common dirtbag thief would have on him.