^That is true, AM3 processors have both memory controllers, (DDR2 and DDR3) they will work in most (most) any AM2+ motherboard, and they will work with either DD2 or DDR3 memory.
But you cannot use a AM2+ processor in an AM3 board. The AM2+ processors only have the DDR2 memory controller, and so they will not work on an AM3 board that requires DDR3 memory.
That is the difference between the 2,, the sockets are actually the same.