Actually, there is a bit more to it than someguy7 said. According to Wikipedia at http://en.wikipedia.org/wiki/Socket_AM3,
Socket AM3 breaks compatibility with AM2/AM2+ processors due to a subtle change in key placement. The AM3 socket has 941 pin contacts while current AM3 processors have only 938 pins.
They even include a reference to a trial done by Tom's Hardware, so it might be worth your while to read the Wikipedia article.