Officially IB supports up to 2133Mhz and in-officially pretty much anything.
Yet I doubt it will ship with anything faster than 1600.
On the CPU side anything past 1333 is pretty much wasted energy even for the Quads in 95% of all workloads.
Only the GPU likes bandwidth up to 1866 quite a lot, but that really only matters for the Dual Core ones without dedicated GPUs. Given that 1866 needs a bit more power and costs more it is doubtful to make it in either the small 13"(because of cost and power waste) nor the 15"(because of power waste).
1600 DDR3L is the best they could do. Even 1333 1.25V LV wouldn't be a bad idea in the thin Air. There are no LV 1866Mhz modules available yet afaik.
Soon you can probably upgrade them with even 2133Mhz modules. They will probably be available before the world ends in december.