Interesting.. What if the federal government moved to Denver?
I'm guessing most people here have heard of or thought about the Grandfather paradox.http://en.wikipedia.org/wiki/Grandfather_paradox
Something I think about with precognition (via a time traveler or otherwise) is whether or not "the" future can be altered in light of said precognition. There is an argument there that indicates that an omniscient being has no free will because it knows
what it will decide to do forever.
There is an alternate theory which resolves the paradoxes that come with time travel and precognition. Parallel universes.
Supposing there is a universe just like ours except it is running 50 years in what is for us the past, then a parallel jumper could go to that universe, kill "his" grandfather, and there are no paradoxes. Precognition is a glimpse into one slice of the sea of potentialities, perhaps one of many parallel universes. The precognitor could then change what happens in "his" universe.