Well, the 680 was likely going to be released as a 670 Ti, i'm pretty much convinced of this now (after considering the evidence), which is also why the performance difference is so less b/w the two cards.
There's a good chance they have the "true" 680 in the works, but they probably don't have it ready yet, and they wanted to look good at launch.
Also, seeing that the 680/670 are based on GK104, i have a strong feeling that they'll re-release the 680 as the 760 Ti and the 670 will be the 760, just with the names GK114. I somehow doubt that GK110 will come out of hiding before the 780 is launched. Just like what they did with Fermi.