Yep, I would have to agree with Wildeybeast on that one. A Space Marine Codex is almost always released with a new edition.
A huge portion of 40k sales comes from Space Marines and they will always be GW's poster boys, and any changes in the core rules will be capitalised on in the new Dex.
That would mean that if Eldar were coming it would have to be in Q1 or Q2 next year, which just seems too unlikely to me.
Of course I will look silly if they do appear early next year, but I have seen no solid indication that they are even out of the concept stage just yet.