not sure, but i think significant enough that it really HURTS! (i.e. in general the severe pain tells you if it is potentially harmfull)

there was some study a while back where they attempted to show that the small shakes and rattles that you get from "normal" riding of a mountain bike off road (imagine a wash-board section of dirt road going downhill) caused "trauma" signficant enough to cause infertility problems but if i remember right the study was not re-produced by data from anyone else...

--->the last 5 years or so i have riden about 5000-8000 miles a year, mostly on the mountain bike. while i pay attention to my sit-position and saddle choice (i spend a lot of time on it) and i try and come out of the saddle evey once and a while on a long climb, i don't have any major concerns here (i once had a saddle where i experienced some numbness i the area, so i changed my saddle) --- i do have a foam pad over the top-tube of my mountain bike which could reduce the severity of a crotch-buster but it is really more there to reduce knee injuries when i do technical riding and jumping (i also wear knee/leg-pads but the top-tube pad adds additional protection and it is always there even when i don't have the pads on)