In alot of the fermi threads there is speculation.
Some say it will OC like a champ. I think their reasoning was It has similiar temps as GTX280 and it oc'd well enough (I don't remember where they got the GTX480 temps though, if that was their reasoning, I forget)
Others say with the kind of cooler it needs, it will be super hot, and won't OC at all.
I think I'll wait for user reviews myself