Some resorts let you skin uphill sight unseen, but in a disturbing trend, a few have sought to ban uphill travel. Tiny Sunlight ski resort, out on the Western Slope, came up with a middle way: they let you skin uphill provided you register for a free hiking pass.
Lou Dawson over at WildSnow worries that resorts might start charging for uphill passes, but as long as it's reasonable, I could get down with it.
Here's a starter list of a few resorts and their uphill-travel policies:
Sunlight: Free pass
Crested Butte: Free pass
Breckenridge: skiers must start before lift-running operations
Vail: Open, provided skiers follow guidelines
Winter Park: Not allowed after the end of ski season.
Eldora: Not allowed
When in doubt, call the resort for info. Happy skiing--there's only a bit left.