I just ordered a new 2010 CTS 3.6L FE3 and should take delivery around Christmas (I hope). I will have to swap the 19" summer tires for a set of winters. Can anyone tell me if it is possible to go to an 18" rim or is the rotor diameter an issue?

Wish I had the FE3. I have the 18 inch wheels. I think you'll be ok going down to 18 inch but not lower (because of the size of the brakes). Let me know what you find in the way of winter tires. I'm considering the opposite - buying 19 inch v series rims for summer and using my stock 18 inch rim for winter tires.

I just ordered a set of 18"s from Tire Rack. They assured me that they would fit or they would take them back. Good enough for me.