DDR2 will be the norm for a while yet. Certainly until DDR3 prices are able to compete with DDR2.
It's different from the changeover from DDR to DDR2, when Intel dumped DDR from its chipsets. Back then DDR couldn't keep up with the proposed 266mhz CPUs (well it could, but it was quite rare and pretty expensive), and DDR2 was going to be needed.
DDR3 won't be *needed* until either Intel or AMD release a 533mhz CPU. And that's going to be a few years yet.
The only reason we will switch to DDR3 is when it's just as cheap as DDR2.