I"m shooting a wedding tomorrow in Vermont and am worried about giong from the cold outdoors to the warm indoors. Are P2 cards as vulnerable to condensation as tape media?

In theory, no, since the cards themselves are a sealed enclosure. The bigger concern is the camera itself, not the cards especially since they are inside the camera. If you're concerned about dew/condensation you should have a cover on the camera; if you don't own one a plastic garbage bag will do.

The HVX is rugged but not weather-sealed like a pro 35mm SLR, and I'd be concerned about the buttons on top, the gap between the EVF and handle and other small places that would allow moisture droplets to fall inside the body or lens housing.

I was just dealing with this same issue this morning as I wanted to go outside and shoot the unusual fog we had this morning.

The thing you should worry about is the lens fogging or worse - any interior parts of the optical system. All of it will spoil your image for quite a while.

If you are outside in a snowy landscape and then enter a room packed with dancing, sweating people, you will be in trouble. If the camera's cold, avoid sticky hot rooms. Try to find a room "in between" where you can warm up the camera a couple of minutes. Going from cold into a church should give you no problems.

If you're lens has fogged it can take you up to 30 mins. before it gets clear again.

I use the trash bag technique as well, to put a dry-air buffer around the camera before bringing it in from the cold. A few minutes like that lets it adjust without getting condensation on the camera.