I saw Henry Cole interview one of the past Finnish rally world champions (can't remember his name) who is now into bikes, he has a garage full of KTM's. Anyway he was saying that spiked tyres on ice are more predicable and grip better than knobblies on dirt. He said the the ice always has the same consistancy whereas the dirt changes all the time.
I would imagine he knows what he is talking about; but for somebody who has never ridden ice before I suppose they would have to recalibrate their brain first.