In reply to Toby S:
I met a parent once (before the internet was quite so much a part of everyday life) who basically said to his teenage "son here's how you find porn, go have fun". After a rather intense couple of weeks the lad got generally bored of it all and went back to more innocent computer pursuits (killing large numbers of random animated strangers and so forth).
We've not quite gone this far with our kids. We have, though, tried to work on the principle that them developing their own media-use discipline - which they are going to need throughout life - is as important as protecting them from nastiness.
So we don't use internet filters (I did when they were under about eight), applied some sensible restrictions about when they use it (two or three days a week are no-computer days). All of the family have their own accounts on all computers and most computer use is where everyone can see it.
I'm not sure we have it right, and I'm interested by this thread, but our now-adult oldest son doesn't seem too deranged.
Paper-based media is quite another matter, however. Just how many vampire-based romantic fiction can the average 11 year old consume without it doing permanent and lasting damage.