I've had the same thoughts since going from the 737-300/400 to the -700. We have 3 so far and all 3 need a bootfull of right rudder in the cruise. As it's on all 3 I don't reckon it's a rigging problem, and the fueselage of the -700 is the same as the -300, so I also put it down to P factor. Any other ideas ?