People seem to be forgetting that an implementation includes an execution environment as well as just a compiler, and that a perfectly valid (and, to my mind, very much recommended) option for undefined behaviour is to behave in some documented manner characteristic of the platform. I don't see this "no obligation" stuff as a great idea, not when it results in so much surprising behaviour - and especially not for simple situations where there's something obvious that the compiler could do, that would surprise absolutely nobody familiar with the system in question.
I always imagined undefined behaviour was there to avoid tying implementations' hands, by having the standard not mandate things that vary in practice. But it seems that people are assuming it's there to give compiler writers carte blanche to do whatever they like, and then point at the standard as justification. Given how much stuff could potentially be added to C to improve it, I don't know why people are spending all this time trying to figure out all the ways in which the letter of the law allows them to confuse the programmer.
See also somebody else's rant about strict aliasing: http://robertoconcerto.blogspot.co.uk/2010/10/strict-aliasin...