Quite simply, with max settings applied the 680 is faster within most games, both at stock and when overclocked. According to every review so far, the 680 averages 10% faster.
It costs less (but still too bloody expensive), offers more features, requires less power, and even overclocks just as well as the 7970's. NVidia have just beat AMD's top part with their mainstream performance part. Until today people could argue that the 7970 was a good part. GK104 performance and features make the 7970 look very poor indeed. Even the ~1% of gamers who use 3x monitor setups will be happy because NVidia now support up to 4x.
I cannot wait for NVidia's GK110 high-end part as it appears to be GK104 +50%. Maybe someone should start a new thread.
I've only looked at Overclock3d's review on the GTX680, but they both seemed about level pegging, not a 10% performance difference! The 680 edges it in some, the 7970 in others. Where it does do better is power consumption.
According to the roadmap on Techpowerup the GTX680 is their premier single GPU offering until late 2012/early 2013.
To be honest, with two GPUs that are similar in performance/price from two rival companies I'm looking towards the price war, been 2 years since I made any upgrades!