Well that's it, isn't it?
I weigh 135-140lb, give or take, depending on my hydration level. By the end of a week's commute I may possibly be down around 130lb. IOW, I can get away with Carbon whatever. I just don't stress the bike much at all even barreling into potholes or dropping off curbs, so I ride Carbon with very few qualms. That Carbon fork still freaks me out sometimes, though. It's the failure mode: yikes. If I weighed much more: I'd be riding something else.
I was told recently, by a guy at the LBS, that 185lb is the Carbon warranty cut off for Trek.