FUD Zilla writes the upcoming AM3 processors from AMD will work in existing AM2 motherboard:
AM3 and AM2 sockets are pin compatible, but naturally some of the AM3 pins are for DDR3 memory while in AM2 they are for DDR2 memory. Since Deneb and later Propus and other 45nm CPUs have a combo memory controller, the CPU will be able to support both DDR2 and DDR3 memory.
Our sources said that the original plan is that AM3 CPU will work in your AM2 motherboard with DDR2 and once the right time is come, you can plug this CPU in AM3 board and enjoy the pleasure of DDR3 memory.
The first 45nm quad-core Deneb chips are expected in Q4 2008.