Cycling on Ski trails

Me? for exercize, I don't know why anyone would bike on snow rather
than ski, but a lot of people like to do it in the winter. I just
don't enjoy it at all. Transportation is a different story. I love to
use my bike for transportation in any conditions I need to, but not
just for recreational purposes (in the winter, on snow).

Agreed, I'd rather be on skis as well, however there seem to be some
di-hard bike folks who MTB year round. For those who are seriously
interested in snow biking there is this:
http://www.surlybikes.com/2004_09_01...ogarchive.html

It's a prototype frame, but the rims were designed a couple of years ago
to support folks interested in adventure races like the Iditasport.