The Advantage of AM2+
AMD has been using a fundamentally similar socket for some time now. Arriving approximately two and a half years ago, socket AM2 replaced Socket 939 and Socket 754 processors from AMD. Recently, Socket AM2 has been changed to Socket AM2+, which has added new features and improved the speed of AMD's HyperTransport technology, and in the future AMD will add socket AM3, which will add more features. There are some incompatibilities between AM2 and AM2+ processors, depending on your motherboard and the processor in question. The largest incompatibility seems to result from the high power draw of high-end Phenom and Phenom II processors, which exceeds what is supported by some older AM2 motherboards. It is likely that some other incompatibilities will result when AM3 debuts, as well. But fundamentally, all of these sockets use the same shape, the same number of pins, and the same basic features.
This gives AMD an advantage over Intel. Intel's Core i7 is brilliant, but it uses a new socket which is in no way compatible with previous socket LGA 775 processors, which would include the Core 2 Duos and Core 2 Quads. For the person who is considering a brand-new system, this is a dilemma. Buying an LGA 775 motherboard and an older processor will be relatively cheap, and will perform just as well as a Phenom II system with an AM2+ motherboard, but any future processor upgrades will require an entirely new motherboard, making it no simple upgrade. Buying Core i7 will give the user a good choice of upgrades in the future, but is extremely expensive.
AMD doesn't require a compromise. By building a new system around a Phenom II, the user will have both a processor that is a good value, and a motherboard that will provide a clear, if less than earth-shattering, set of upgrades until at least 2011, when the next major architecture change comes down the pipe from AMD.