On the off chance that you live anywhere that actually still gets snow (it's looking like the temperature is going to be in the fifties here in Michigan for Christmas) you may want to check this thing out. Our buddies over at Engadget
spotted what looks like the spawn of a three-way between a modern dual suspension mountain bike, a WW2 German half-track and a snowmobile. Maneuvering this thing through snow looks like it might be rather challenging, but if you're interested, KTrak
is offering the kit at dealer
pricing of $300 until the end of the year. After that the rear drive track kit will list for $400 with the ski kit another $140 in adult sizes. They also offer a kids sized kit at $200/$90. It appears from the site that the kit should fit on any modern mountain bike but your mileage may vary.
[Source: Ktrak via Engadget