HKEPC has published the first photos of the the forthcoming Phenom II AM3. On the left, you can see the back of a Phenom AM3 and on the right a Phenom AM2+. You can see that 2 pins present on the AM2+ are not on the AM3.

In fact, it will be possible, as expected, to use an AM3 CPU on an AM2+ motherboard, but not vice versa.
To recap, the Phenom II AM3 will be launched in February, while the Phenom II AM2+ is expected to be out in January.