To be completely safe it needs to seal, since the idea is to completely isolate your camera from the inside air so that it can live in the same volume of air and not have wave after wave of water vapor condensing on it. Try it with your favorite cold canned drink first if you're worried. You should be able to get it all the way to room temperature without any condensation forming on the can. And if you time how long the drink takes, that should be a pretty good estimate of how long to wait for your camera.
I should mention that I take my camera out in below-freezing temps almost every day in the winter and don't have problems with condensation if I just leave it in my camera bag until it's up to room temp, so maybe you don't need a sealed bag.

Thanks guys. So do I need a ziplock bag (something that seals) or just a plastic bag that i can tie shut around the camera so the condensation forms on the bag, not the camera?

thanks again!

Inside to outside should be no problem, since any water on the camera will evaporate off because the camera is so hot compared to the surroundings. It's outside to in that you should worry about, and the ziploc bag is a great idea. Zip it up while it's outside and give it enough time to come to room temperature after bringing it in.

Don't snowboard and take photos at the same time, and be careful of condensation when moving from outside to inside or inside to outside. If you can, either leave all your gear outside, or leave all your gear inside. If anything, I hear that if you're going from inside to outside or outside to inside, get a ziploc bag and keep the camera inside of it for a half hour or so. Then you take it out. I haven't tried it myself, but it's worth a try.

What camera do you have? If your lens is the Nikon 24-70mm, then it is weather sealed.

Gentlemen,

Tomorrow I am off to Vail to get some nice days in on the mountain. I'm bringing my Nikon (and probably my grandpa's Canon AE-1) with loaded into a DaKine Mission Photo backpack.

I guess I should only bring it out on clear days without snow in the forecast, but I thought about fashioning some sort of long snowproof sleeve that tapes to my lens hood of the 24-70 and extends past the body to keep snow/moisture out.

I'd love to get some good shots, but obviously not at the cost of my camera function.

Any thoughts, opinions, advice?

Thanks guys!