There is speculation about when Blizzard will release Epic gems and how they will be released, but very little about why they would do so.
Answer the why, and I think you may get clues as to when.
For the record, I was not a jewelcrafter when they released epic gems in Northrend, and my first toon reached level 70 weeks before Northrend was made available.
At this point, Cataclysm is harder than Wrath was at the end. Even ICC had been nerfed into the ground, to the point that even a very slow progression player like myself had a shot at the lich king (for the record, I got him down to around 30% the last weekend before Cata hit).
Today, dungeons and raids are tuned to be hard. I have no doubt that when 4.2 hits; the new raids will be just as hard, and will strongly encourage toons to be properly geared, enchanted and gemmed. However, if Rare gems is what we have, then Rare gems will be 'properly gemmed'.
I don't think that anyone is really expecting patch 4.2 to start off easy.
Releasing new gem types (or even enchants) does not make the new raids (or newly released dungeons) easier. When they are released, any new progression content will be tuned to expect them. Epic gems will provide yet another nerf to older content. However, we will all be excited about the new shiny gems, without percieving the nerf.
Blizzard have already stated that they are nerfing the existing raids. I suspect this will be sufficient for their purposes. I suspect we will see Epic gems and new enchants when Blizzard want to give existing content a nerf.
For the record : I do not think that Epic gems will be released on patch 4.2
This is pure speculation. I still have a small stockpile of pyrite ore, and a few unspent JC tokens just in case.