With the unlocked k series CPUs, you can actually overclock with 1333 ram as you typically only adjust the multiplier and not the front side bus. however, 1600 still gives you a little tweaking room if you need it.
Those higher speed rams, especially the 1866, are mainly made for AMD APUs and CPUs to enhance their performance. In other words, it does not really do much for Intel CPUs.
what are the chances that ivy bridge will use higher memory speeds?
The only difference is the IB default, like the current SB-E, is DDR3-1600 vs SB's DDR3-1333. As far as maximum, DDR3-2600 or faster. Folks get tunnel; vision over frequency I on the other hand look at 'real' speed which includes CAS Timings which are frankly more important or at minimum equally important to performance.
My best 'guess' is DDR3-1866 will be the IB 'sweet-spot' with CAS 9.