AMD or the motherboard manufacturers should do some BIOS tweaks to make it happen but the bottom line is that it could be done.
There is a big indication that DDR 3 ram modules might be identical to DDR 2 ones. So there is a big chance that you will be able to plug the new memory in the old slots. Still knowing how the industry goes we could expect that there might be either the new chipset revision or more likely socket AM2 tweak or re-spin to make the support. We know that chipset doesn't have to do anything with the memory but Nvidia did introduce MCP 55 a new chipset that will support DDR 2 capable socket AM2.