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There's a well-written list of a dozen or so reasons your fuel economy drops when the weather gets colder, floating around the forum somewhere, but here's a quick list off the top of my head:
Winter gas has less energy per gallon. The viscosities Aerohead mentioned mean that every bearing and fluid in your car requires more energy to move. Cold air is denser and harder to push out of the way, increasing aerodynamic drag. Your ECU will run the engine richer and probably idle higher. The air in your tires has contracted and you'll have to add more to compensate.
The optimum fuel economy weather is: dry, and as hot as you can stand without using the A/C or opening the windows.
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