Take this for what it is - an anecdotal tale from one person. I ride a 2-years old (2010 MY) Scott CR1, which has a fairly light carbon frame. I weigh 240, and have had zero issues with my frame. The only concession I make to my weight is to try and be out of the saddle if I see a significant bump ahead. (Without really knowing, I think that the top of the seat tube, by the seat post clamp, might represent the weakest point of my frame.)

(I have 20-spoke wheels as well....)