It is worth noting that there are cheaper alternatives to the unobtainable Nokian studded tires mentioned above. I found some more reasonably-priced studded tires at Bike Nashbar for 30 bux a piece. They go under the name IRC Blizzaks iirc. As with all products stocked at BN, they go in and out of stock at random intervals.
Snow biking is the most zen-like form of bike-riding that I have ever experienced. Being in the middle of a large forest, only hearing your jacket rustling and the snow seperating for your tires, is an experience above the par of anything else on two wheels. Snow biking is definitely a one-person activity, because of the speed/flotation differences of people of various weights and sizes.
If you like snow biking, and get used to it, I'd suggest that you try playing the game Elimination on an empty snow-covered field with your biking buds.