I killed a GF4 Ti4400. i have two or three of my friends who have killed a 9800Pro. My cousin damaged a 7900GTX. It still worked but had to be UNDERclocked to be stable. None of the deaths were due to extreme OC's either. They were all operating at stock voltages and the 9800's actually had aftermarket arctic coolers installed on them, so GPU temps were well under control.
So yes OCing can kill a GPU.
So yes OCing can kill a GPU.