I hate to be a pessimist, but living in denver i have never seen United start a fare war. Usually good fares on United are soley a response to other airlines fare wars. For example, UAL just finished a fare sale to the east coast from denver, and at first it seemed genuine, but upon closer inspection i found ATA started the fare war and united just matched.
Oh well, enough pessimism, im flying NW from phx to bos in april for $175! god bless NW! Even better, i get to fly on a DC-10!