Assuming 2.5 dungeons give i100 gear, I imagine 3.0 dungeons will drop stuff from ilvls i110 to i130. Those farming final coil will be ready, those not quite through with second coil will be able to participate still, and those with just i90 stuff will need to do some work to meet the requirements for 3.0 dungeons.

Also, I thought YoshiP had said something in a prior interview that an example of existing gear with a level cap of 60 would work something like "gear level is 50 = character level 60 = -i10 from said piece of gear". So our i130 shinies will seem like i120 at level 60 and so forth.