The case of "It hasnt happened to me, so theres no way it could have happened to anyone else on the planet" sure is strong on this forum.
The back of my iPhone 4 scratched so severely that I had to have it replaced because any photos were just a haze of scratches, just from being in my pocket with nothing else, its why I have a huge chunky case on my 4S.
The iPod touch is very strong, actually. It's lighter and doesn't have the glass all around it. Still, a thin iPhone case is good enough.
A lot of people at my school broke their iPhone 4s (not 4s, plural of 4) on the retreat because they were very stupid and put them without cases in camping bags that got smashed and stuff.