AMD basically told everyone about socket 939 right after the A64 was lauched, and said it would be the long-term A64 platform. 754 was destined for lower end chips from the get-go. Yeah, it sucks that 939 wasn't ready by the time the A64 launched, but it's not everyone didn't know it was coming.
And yes, if AMD releases DDR2 (or 3) support it will need a new socket. Rumors of "Socket 900" and "Socket 1207" have been floating around for a while. Of course, it could always be waiting for FB-DIMMs, likely in the form of DDR3, before it makes the transition, but who know. Regardless, DDR2 is still a long way off, won't make much of a difference in your current hammer platform anyways, and probably should have no bearing on your computer buying decision right now. If you wait for it, you'll be waiting a long time, and if you buy an intel system simply because you HAVE to have DDR2, then you will end up spending more money for lower performance and less features.