It’s been done, and before any nerfs were applied to the difficulty. Diablo 3 user Kripparian, along with a single ally, has defeated Diablo on Inferno difficulty with a hardcore character. They’ve been confirmed by Blizzard as the first two to successfully accomplish the feat, and I don’t have much else to say aside from “well done!” Inferno is certainly no cakewalk, and these two should be proud of their deed.
If you’d like to see a video of the Diablo kill (there are a couple of close calls!) check out this youtube link.
Congrats to the both of these guys!