After a little fact checking (on the part of Blizzard) and some discussion about if it was a legitimate claim, Canadian Diablo III player Kripparrian (with help from wizard friend Krippi) is officially the first person in the entire world to complete Blizzard's RPG on the Inferno difficulty level. He did this with a Hardcore character to boot, which means that the game ends if you die.
Admittedly there was some doubt within Blizzard that Kripparrian was the first player in the world to accomplish this monumental milestone, but a tweet from Blizzard community manager Bashiok confirmed it:
"My apologies Kripp and Krippi, looks like you were 1st HC Inferno clear (my spreadsheet deciphering isn't what it used to be) Congrats!," he said in a Tweet.
The recent 1.0.3 patch has made Inferno mode a lot easier, so Kripparrian's achievement is all the more impressive. You can check out this milestone in the video to your left.