I've checked my camera housing in some cases, and carried it on in other cases. I tend to carry on the camera itself, as opposed to putting it in the housing, because I don't trust the baggage handlers to be gentle with my bag. Going to and from PNG, I had to carry the housing and camera on, because I would otherwise have exceeded the weight allowance for checked baggage. Even if you have a lock, its not proof against theft - TSA cut my lock open the last time I traveled, since I didn't have a TSA lock. If the baggage handlers are going to steal something, they'll be able to steal it, and I suspect you're only recourse would be insurance if they chose to.
I prefer to carry things on in a roller bag if possible, my Patima housing and Nikonos strobe barely fit the bill for that, and it makes for a fairly heavy carry-on. I'm more worried about rough handling than theft. On the other hand, I've accidentallyt dropped my aluminum housing from 5 feet up with the camera in the housing, and it didn't have an effect on the camera or the housing, its pretty solid. I also had a shuttle bus attendant toss my bag about 5' with my housing and camera in a bag, again with no effect. Not what I want to see on a regular basis, but at least its given me confidence in the toughness of my housing.