How about anecdotal evidence? 
  
Years climbing: 13 and change 
Bolts fallen, hung, and repeatedly yarded on: too numerous to count 
Bolt failures experienced or witnessed: zero 
Bolt failures heard of: a handful, in Thailand, due to now well-publicized corrosion 
  
Obviously, ancient alpine bolts might be a different story, but if it has a buttonhead on it, you can reasonably guess it's less bomber than a Rawl 5-piece.  
joey runs into them all of the time...in fact, he just removed one with his teeth and upgraded it to a cracked eye kb, just to keep the sporting side of it intact...