Depends on your definition of locked. Palomino's don't have an adjustable multiplier unless you connect the L1 bridges... same with T-Bred A's. However, T-Bred B's and Bartons have adjustable multipliers (12.5 and below) in nForce2 motherboards. Certain nForce2 boards like the Epox 8RDA+ allow multipliers above 12.5 to be selected because they have a 5th multiplier bit.
The new Barton XP2500's, week 40 and newer, have the multiplier locked and it can't be changed no matter what motherboard you have or what you do to physically modify it.