Details released to OEMs suggest that AMDs upcoming AM3-based CPUs will both fit into and work with motherboards using the existing AM2 socket. This is good news for users that have motherboards supporting the AM2 socket because they will be able to switch to AM3 CPUs without having to upgrade the rest of the hardware (in particular the motherboard and RAM).
I have to admit to being somewhat surprised by this move. An older socket supporting a more AMDs upcoming AM3-based CPUs will both fit into and work with motherboards using the existing AM2 socketmodern CPU is very unusual since the normal maneuver is to upgrade both CPU and the motherboard (not to mention the RAM too). Given the fact that AM2 will be forward compatible with AM3, enthusiasts will only need to replace the CPU to make the leap into the new technology. This is also good news for OEMs, who will be able to slot AM3 CPUs into existing lines when they become available (sometime late 2007). This also means that the pin-outs of AM3 CPUs are going to be similar to AM2 but different enough so that AM2 CPUs wouldn't fit into an AM3 socket (one fewer pin would be enough ... hmmm 939 pins ... a task that's easy to achieve given the number of grounds on a modern CPU). No matter, it's still a good thing and we could be seeing the beginning of a cycle where a socket will support two generations of CPU.
To pull off this trick, AMD will have to include support for both DDR2 and DDR3 in the integrated memory controller, as well as having HyperTransport 3.0 (HT-3) support earlier revisions of the bus protocol too.
However, there is a catch. AM3 CPUs fitted onto AM2 motherboards will only support DDR2 and won't be able to take advantage of HT-3, but I don't see this being a huge problem since neither are likely to offer much in the way of benefits for desktop users.