Over the next two years we're going to see four new sockets from AMD: the AM2, AM2+, AM3 and the Cerberus:
AM3 is the successor to AM2+ (duh), and it arrives in the Shanghai/Budapest generation. AM3 chips have HT3.0 and force a split voltage plane. You can do all sorts of tricks with HT3.0 that you could not do with HT2.x, so this will be a major step forward, but the pin count should not change.
You can plug an AM2+ chip into an AM3 socket, and plug an AM3 chip into an AM2+ socket. Again you do not gain functionality going forward, and you lose functionality going backward. This assumes that the AM3 boards do not use features that make the AM2+ chips choke like splitting HT links. While I have not heard for sure, this AM2+ chip on advanced AM3 boards does not sound feasible, so I will go out on a limb and say it won't work.
AM3 forces split voltage planes, where it is supported in AM2+ and not supported in AM2. This means AM2 and AM3 sockets and chips will not play well together no matter how hard you push them into the socket. AM2+ chips will go both ways, pun intended
AM2 is already available and future AM2+ processors and sockets will have support for AM2. AM3 processors and sockets won't support AM2 and the Cerberus which will arrive in late 2008 will use a new physical socket and won't be backwards compatible at all.
Remember I said something about variants? At the very least, there will be an A64 socket and an Opteron socket, so that means 8 sockets in 2 years, one a quarter. Lets not forget laptops, the occasional 4x4 tweak, and who knows what else. This means that the staid old AMD socket that never changes will change a lot.