I've been thinking about the original question on the aerodynamics in the slip. In the 172 when you slip with full flaps you get an oscillation. I believe the reason for this would be the airflow would be displaced infront of the elevator making it ineffective causing the nose to pitch down. This would reduce the angle of attack, moving the airflow back over the elevator causing the nose to pitch up again, once again making the elevator ineffective.
The only reason Cessna would care about this since you can't stall and spin when your'e in a slip since you don't have enough elevator available to cause a stall would be in the event you needed immediate use of elevator for some reason (ie. traffic avoidance) it would not be effective. Any thoughts on this idea?