Seely is (mostly) right. But I fancy Tioga Bloodhouds (agressively knobby 27x 1-3/8 inch cyclocross tires)
http://www.mtbr.com/reviews/Tire/product_23336.shtml
Tioga Bloodhounds keep my Marin Sausalito (and me) upright on most snowy trails and roads.
http://www.sheldonbrown.com/harris/tires/630.html
But I don't think I'll be waving to you when I'm Telemark-skiing A-Basin in early March 2005.
Then again, who knows? A-Basin inspires the extreme ;-) You could be floating on powder on a Santa Cruz Blur with 26x2 Hutchison low-pressure tubeless Scorpions. (Low pressure and moderately aggressive knobs, hence more friction on snow- I think; -).
We need a new thread on this topic....We're not talking about the 700 x 35 C Nokian Hakkapelitta TR1110 studded tires for snowy/icy roads- that's a no-brainer. We're thinking semi-aggressive knobby, low-pressure, and NOT sinking-in-snow off-road tires. Any one from Finland or Norway found a bike/tire combo (no reindeer, please :-) that drives through snow without exceeding your aerobic threshold and without experiencing that sinking sensation?
Maybe researching winning tires/bikes that have performed well in Iditasport (extreme) conditions will narrow our choice of off-road snow tires. Or maybe not. What I have found so far is what worked well in 1996-2000 Iditasport (bike) races. All this was pre-Hutchison and pre-Tioga R&D.
http://www.allweathersports.com/isport/ibiketips.html
Another source of info may be the (extreme) Snow King World Cup [bike] Race held in Kvitfjell, Norway on 10 February 2004.
http://www.worldcup.kvitfjell.no/ep/18/?articleid=19
Maybe Siemens (who sponsored the race) will outsource a "Siemens Snow King" MTB snow tire to (for example) Continental ;-)