If it's who I think you're asking about, that's Diablo's new look. But just to be sure, do you have a link to a picture?
Yes the cinematic demon infact Diablo who's been new appearance confirmed through a couple sources and have a presence in the game.
- "Victorious Barbarian standing atop a defeated Diablo shouting towards the High Heavens" (2009 Barbarian statuette over the D3 cinematic trailer roaring demon's skull), this was the first real confirmation the red demon being the Lord of Terror.
- And there http://www.diablofans.com/news/863-it-ends-with-diablo/ where a Blizzard employe confirm Diablo appearing in D3.
- Finally there is the Son of the Storm (official Blizzard site for their artists) artist "Glowei" D3 piece seen previously back in Blizzcon 2010 http://sonsofthestorm.com/viewer.php?artist=glowei&cat=diablo&art=6 titled: Diablo - Lord of Terror