Illustrator CS5 Perspective Grid: Locking the Perspective Grid
After the perspective grid is set up just the way you want it, you want to be able to freely move the Plane Control Points, but not to move the Grid Control Points. Illustrator provides an easy way to do this through the Lock Grid command. [View > Perspective Grid > Lock Grid].
The Lock Grid command locks those control points that would change the perspective, while letting the others be adjustable.