Winter commuting is SO MUCH FUN! In my opinion, all you need is a really good set of lights, and some semi-slick tires. A yard-sale 26" MTB can't be beat for winter commuting. Oh, and some cold-weather clothing helps greatly (full-finger gloves and warm socks are a must). I'm already looking forward to cold weather.
In response to the OP, yes, I have expanded from my usual 3 days/week to 4 or 5 days in response to gas prices. My car has a 12-gallon tank, and I fill it once per month. Right now the gas in my tank (3/4 full) is $2.79 gas. Current prices are $3.19 in my area.