Both the Iditarod and the Susitna are cool races. The weather is the major factor on the final outcome between the cyclists, skiers and runners. We have a lot of ultra wilderness races winter & summer here. It brings some major contenders in from all over the world.
Some years the bikes have to be pushed through deep snow for miles on end due to the outrageous conditions. The skiers seem to think it's unfair to ride a bike when it's hard and flat, and/or the runners may be subjected to a hundred miles of post holing
I've considered riding the Susitna, but haven't tried it yet. I'm usually skiing throughout winter, but this year I may be riding more.
I'm planning on organizing a metric century here for the winter... I figure it should be a blast and many folks I know are interested in doing a ride like this.
Mind you...Canadians, despite being nice people, are also a little nuts.
You should consider doing the Century-A-Month challenge. I did it in Winnipeg in 2003, and then in Red Deer in 2005 and 2006. I took a break from it this year, but am thinking of starting again in January 2008.