A redditor going by "CD Productions", a reference to the protagonist in GTA San Andreas, made a post on the GTA 5 subreddit to state that an actor named Steven Ogg will be playing the role of Trevor, one of GTA 5's three main characters.
He certainly looks the part, and his voice can be heard in the short film Disgrace, in which you can hear him yell and sound a lot like Trevor. Watch the video here.
Some quick research will bring up Steven Ogg's official website and portfolio, at StevenOgg.com, which shows that he's done a lot of voice work for advertisements, narrated a documentary, and done some bit roles in TV shows like Law & Order. He's no stranger to video games, and starred in the game Alone in the Dark.
Keep in mind that none of this can be proven at the moment, but the evidence seems solid. Only time will tell if Steven Ogg is playing the role of Trevor in the game.
The would-be reveal follows in the wake of the discovery that Jeff Wincott will be playing the role of a supporting character named Casey, who has since confirmed his role.