Truer words were never spoken. My commute is 10 miles. In cold weather, I spend the second 5 miles trying to overcome the lousy mileage I get the first 5 miles when the engine is cold. If my commute were 5 miles further, with similar terrain, I'd be a hypermiller!

It's not as noticeable when the temps are warmer, but it's still there. The larger the percentage of your commute time is spent in warmup, the lower your mileage will be (in general).