I just spent a whole lot of time working out whether I actually did ride my bike slower with my studded tires during the winter than on my road tires during the rest of the year. (Note: I ride only one bike, but I change the tires for the winter.)
Short answer: yes.
Longer answer: Examining the GPS-gathered data from mid-January 2011 until the end of March 2011, I was able to make two datasets. From January 2011 until mid-February 2011, I was riding on studded tires, and since mid-March, I was riding on road tires.
The total distance traveled from mid-January until mid-February was almost identical to the total distance traveled from mid-March until the end of March. (Yes, I didn't ride as often whilst snow and ice covered the ground; best to be safe than sorry.) The average speed during Jan/Feb was 8.9 mph. However, the average speed during mid-to-end-March was 10.1 mph; 1.2 mph difference.