If you want to get any volume production and to have at least hinderers of modules you won't be able to go over 1300 MHz.
We spoke with numerous of memory manufactures and this was the realistic number we got. Corsair demonstrated rather modest 1315 MHz with its Dominator, Patriot can run 1350 at special lab conditions while OCZ claims it can go as high as 1400 MHz.
All this is possible but there will be a few out of the thousands modules that will be able to work at such a high speed. So this is a showing off marchitecture and you should realistically settle at 1300 MHz.
If you use water cooling or something even more extreme there might be a chance to go even higher but not much higher than 1350. One thing is to boot a machine at 1333 MHz, we did it in a lab a while ago but it is completely other story where you have to run a machine stable at this speed. It is very hard but not impossible with super cooling.
DDR 3 gives us a hope as this memory will mess with the latencies but will also get you to DDR 3 1333 and even DDR 3 1600 speeds at 1.5 V and certified by JEDEC.