I heard something about Rivatuner being able to overclock the shader segment of the G80 core, but since the latest version was out before the G80's, it's kind of hard to imagine thats true.
I've talked to the creator of ATI Tool and he says that he dosent know how to overclock the shader segment. Perhaps they will all figure it out at some point and we will be able to overclock.
Dont be too sad though. The lack of G80 support right now will not go on forever. The 8800's are the only graphics cards wich dont have aftermarket coolers available, but that will soon change with the amazing HR-03 Plus. And Im sure that we will be able to overclock the shader segment at some point.. its just a matter of figuring out where the heck it is :P