04 transmission in an 03

Has anyone heard about swapping the transmission from an automatic 04 with the steptronic into an 03? What kind of costs would be involved?

I know the 04 is a bigger car, but do the trannies match up? I would go with a 6-speed from an 03, but I do quite a bit of rush hour driving everyday. The Steptronic from an 04 seems like it might be a better (more fun) time.