Actually, I've been wearing cross country ski boots to ride to work for a couple of weeks, now. They're the old three-pin binding style, so the main difference between them and boots is the longer, square toe. I have longer clips on the pedals than I need for my foot size, so they fit quite well. I have two pairs of these boots, but I've decided the newer, synthetic ones are too waterproof and fit too tight. I've switched to some oversized ancient leather ones that are quite comfortable. (Except on the ride home, cause it still gets up into the sixties in the afternoon. That's why I don't wear the plastic ones. They stink.) And since I can wear more than one pair of socks, I'm hoping they'll be good down to zero or so. If it's raining, I wear my gaiters to keep water out of the boots.

I find that what I wear when XC skiing isn't wind resistant enough for biking. I'd just wear nylon pants over thermals and a t-shirt and sweater for skiing. But on a bike, the wind chill makes anything cotton too cold. (I always sweat when I ride, even if most of my skin is freezing already.) It hasn't gotten much below freezing, yet.

I've been wearing biking tights with non-biking synthetic long underwear as a base layer.
On top I wear a jersery and arm warmers under fleece or synthetic sweatshirt, and a homemade fleece neck gaiter on colder days.

I've never biked through winter before, so I have basically gone through my wardrobe (and my wife's) trying different kinds of clothing to find what works. As it gets colder, I'll probably have to buy a windbreaker. The Pearl iZumi waterproof/breathable jacket I have is not breathable. It's made from eVent fabric, but I get soaked with sweat whenever I wear it.

XC ski boots look really lame with tights, but after a few years of wearing spandex I don't much care if I look lame. (I just have to stay close to the bike.)