It'd different with the 680 then the 580 though. It's not just a voltage limit you can't change the voltage at all. All you can do is force a constant 1.175. All the aftermarket cards so far have had the same limit but I think I saw a story about a Zotac card of all things that is going to change it.
Anyway though this is the reason why the 7970 keeps up so well with those max OC tests. If Nvidia hadn't done it the 680 would have blown it away as you can see right here with what the kingpin guy did.