Likely in 2000 Bush got an artifically high result there because Cheney gave him a big boost. This year the Cheney effect had worn off.
Yeah, but 2000 was the first time that Teton was the Dems' best county in the state (see my thread, Best Dem and Rep results, by county ).
As for why I'm not quite sure, but obviously resort area workers turned out heavily for Kerry. Since many of these people are pretty young and have relatively recently moved to wherever they are now, I'd figure it's got a lot to do with Dem registration drives since and low turnout among 18-30 year olds in 2000 - much like the equally pronounced Dem swing in college town-dominated counties.
People who live in ski resort areas have to be wealthy enough to afford to live in a place where their aren't jobs that can pay for the cost of housing, or have jobs that aren't in the area (telecommuting or executive positions), or persons who are indifferent to their economic postion (willing to pay outlandish rent for sharing a tiny apartment, so that they can ski, and wait tables at night).
Many of the workers in the ski resorts are seasonal and might not have been able to register before the election.