Basically it is the same situation as when DDR2 first came out. It took a good 12 months for it to be more mainstream in use and was much more expensive when it came out with no real performance gain. Now DDR2 clobbers up on DDR, I am going to assume the same for DDR3, that it will take some time to come down in price and to be better utilized (and to lower the latency).
I know when I built I thought about the dual mb's that had 2 slots for DDR2 and 2 slots for DDR3, but I just went with DDR2 as I figured I would up the whole thing instead in a year or so.
“I'm the decider, and I decide what is best.” - GWB, 2003