I don't know about taboos, but I certainly would be suspect of any such limitations. Literature should challenge our concepts, and create bonds with characters so we can empathize and understand situations unlike our own. So why have subjects that are off limits?
Personally, my WIP deals with an almost rape. I was just asking reader comfort level in the romance writing section, since mine flashes back mid steamy scene. The overwhelming response I received was that if the end was satisfying enough, and you moved with the character from pain, to healing, to healed (or as much as reasonably possible) than readers will forgive most forms of MC torture.
As a reader, I would agree. I can stomach reading about some pretty nasty things, as long as the bad guy gets whats due him in the end.