Just to follow-up my comment on Craig's earlier posting, I thought I'd provide a little more information on snowshoe running.
I started last year after realizing that my time was becoming more and more limited. However, and somewhat contradictively, I wanted to go the summer of 2007 in the best condition I've ever been. I succeeded, and frankly with very little effort relative to past practices.
I ended up trying out snowshoe running and love it (coming from someone who hates running). Actually, it's more like snowshoe trotting because running will kill you in less than 3 minutes. I personally added poles to equation to further increase the cardio effect and get more upper body (UB) workout. Typically bulking up the UB is not ideal for cycling but in this case it's very high reps with little effective weight so mass should be kept in check. Don't underestimate the effect of poles though, your arms will feel like jelly. But the strength you'll gain throughout the winter will pay dividends in technical riding. You core muscles get a great workout as well.
But don't just take my word for it, see what Andreas Hestler has to say (ironically linked from BC Bike race site).
Because snowshoe running is so intense, last winter I only had 2-3 outings per week on the snowshoes lasting about 45-60 minutes each. I supplemented it with 1-2 outings/spins on the bike at about 1 hour each. That's it, 3-5 hours per week and you'll be a hero going into the spring.
BTW, if any of your guys want to try it out, I've got a spare set of running snowshoes you can try. All I use is a pair of Gore-Tex Trail running shoes with cycling booties and you're good to go. Oh yeah, severely under dress b/c you'll be shedding like crazy otherwise... Kind of a nice segway into a naked gear review eh?