A little over a year ago I read in what might have been JP magazine that there was a carbon fiber bumper for a JK from Iceland (which has quite a carbon fiber market) that could actually take a hit, but not recommended for repeated abuse (paraphrasing to the best of my memory). HOWEVER, I cant find the article online any where, but that wont stop me from theorizing IF it can take a good hit then why not? It's 35 bucks less than HALF the price of the genright aluminum alloy bumper, and 165 bucks less than the genright steel bumper...to be fair both metals from genright have a stinger and grill hoop that connect for strength.
Aberle Off Road makes a $700 65 lb cast aluminum bumper that claims to be strong, as it has no weak welding spots holding it together. At any rate, there is a healthy amount of choices you can make, that sure seems like a good thing, but it kinda makes it harder to decide.
http://www.myjeepforum.net/modified-tech-f2/jk-front-bumper-comparison-t152.htm
Note how few bumpers have weight listed, ow well, I still like the idea of aluminum.