I thought I'd share a reply I sent to a PM I got over the weekend:
For safety cables, look for one that will hold at least twice the load your exerting, I shoot for around 3 times as much. When I was doing a lot of rock climbing, I would look at specs for climbing ropes and they quoted these outrageous max force rates, but it's because your not only fighting your weight on the rope, but also gravity and (as a result) velocity. If your stuff falls and only falls an inch, then it's a little over your stuff's weight, but if it's going to call a distance of 2 feet or so before it eats all of the slack in the line, then the initial impact where it tightens up will exhert more force than the weight of the stuff. It would be a hell of a bummer for the clamp/arm/mics/etc falls, the safety cable catches it, and then fails and falls again.
extra points if you can find a cable that is looped on both ends instead of just one with the biner fixed on the other. That way in a pinch you can pick up a $8 locking carabiner for extra protection.
So casually scanning over the B&H catalog, I saw the following:http://www.bhphotovideo.com/c/product/840889-REG/Kupo_KG061112_41_105_cm_Long.html
holds a lot of weight, is fairly long (which is nice for the articulating arms), isn't all that expensive, and has a locking biner on it. I don't personally own one, but I'm tempted to pick one up just cause.