Well, after the long wait to see the holiday schedule loaded, I booked United from PVR-DEN on Dec. 29th. The flights leave within 5 minutes of each other.

Frontier: $537 (plus $20 for carryon)
United: $341 (includes carryon) ^

Frontier just isn't even close to being competitive on a couple other days at the end of the year as well. What do you think is going on? I'd prefer to support them over UA, but not if they are going to be an Ultra High Cost Carrier on these routes. By the way, Skyscanner did not post this lower fare today, but Matrix did. I'm really impressed with that site for finding the best up-to-date prices. If you know of other sites to use, I'd love to hear about them as well.