I'm told that the 37,000 word (per Anderson Cooper on CNN last night) contract you sign when you buy a ticket entitles the airline to give your seat to someone else.
I've never read it, and I never will. So, it may or may not be true. But, according to CNN, United has a legal right to re-accommodate anyone.
This past weekend my Friday flight from Eugene to Knoxville got messed up due to the weather problems, and I was going to have to spend 2 days, yes 2 days in Minneapolis. There were ZERO available flights on Saturday. I asked nicely if they could route me through SLC where I have family, which they did. So, I got a weekend with my parents and a few siblings. If you fly, crap happens. Smile and make the best of it.