HardOCP has the final word on the multiplier locking of the AMD Thunderbird and Duron processors. Here's what AMD is saying

"AMD's Socket A processors (The AMD Athlon and AMD Duron processors) are produced with the multiplier locked and the correct operating frequency laser etched on the processor die. This is done to help deter remarking of processors, which can lead to consumers not getting the product they think they are buying."