Well if you consider that these modules are some of the very first and are still faster than some of the highest end DDR2 I think we can safely say that the technology will allow massive speeds in the near future. That was the way with DDR2 and now we're looking at over double the speeds of the fastest DDR. So you can get an idea of DDR2s life expectancy from that (as long as development trend continue - as they tend to)
We have seen some prices recently for some DDR3 modules in Japan and the prices were pretty reasonable (relatively speaking) so we can probably expect a nice big peak as the speeds go through the roof and then a drop to some more obtainable prices when DDR4 is announced lol
I, personally, am pretty excited, I have a bit of a fast memory fetish