To the best of my knowledge, they aren't. They are the same size and thickness and all that, but the scissor mechanism is positioned differently. (On the Macbook the scissor is mounted horizontally, on the aluminum keyboard, it is mounted vertically.) Exact same mechanism, just positioned differently.
Admittedly, this is comparing my 1st gen MacBook to the aluminum keyboard, I have no idea if the later white macboks are different.
However, in the event that you find that it is the same, drop me a PM, I have a spare topcase laying around, and every key but one is just fine (either the shift or the return key has a crack in the middle)