A New York Post article states:

Despite threats of anti-Prop 8 activists boycotting the heavily Mormon town of Park City, Utah — the festival's recurring location — Sundance plans are still moving forward. [emphasis added]

And here are some stats from a recent survey:

Mormons in the State of Utah – 60%

Mormons in Summit County (Park City) –  28%

Park City is one of the most open-minded, accepting towns I've ever lived in. Although one could make an argument that 28% is a lot compared to United States averages, to refer to Park City as “heavily Mormon” mischaracterizes the nature of the town.

And, if accepted as accurate, the descriptor lends support to the misguided campaign to boycott Utah ski resorts, three of which are in Park City. 

– Mark Worthen