Variety is reporting that their sources have confirmed Josh Brolin has been cast for the role of "Thanos" in Guardians of the Galaxy.
There has been much speculation regarding the galactic-scale super-villain ever since he appeared in the stinger for Marvel's Avengers. First, there was speculation that he was the main villain for Avengers 2 until it was announced that the plot for that movie would follow the storyline for Age of Ultron. Some also suspect that the item Star Lord (What, you haven't heard of him?) is trying to steal in the trailers for GotG is yet another Infinity Stone. Add to that, mentions of Infinity Stones (aka the Infinity Gems in the comics) and the Tesseract (aka the Cosmic Cube) as well as the actual visual of the Infinity Gauntlet in the first Thor movie. Did you blink and miss it?
With Guardians of the Galaxy coming along in the Marvel Cinematic Universe, Thanos was a natural fit for at least a cameo that would lead into a full blown appearance in Avengers 3, which has also been rumored to lead to the death of Captain America. At least, the Steve Rogers/Chris Evans version of him, which would then be taken over by the Winter Soldier/Bucky/Sebastian Stan as has happened in the comics previously. If you don't read the comics, yes, Steve Rogers is currently Captain America. Again. Remember, characters don't stay dead in the Marvel Universe. Unless you're Charles Xavier. So far. Give it a little more time.
*RESUME NON-SPOILER-ish PART*
It remains to be seen how much actual screen time Thanos will be getting in GotG. Newsarama reports that he'll likely be a background character while Ronan the Accuser and his lieutenants take up the main villain duties.
Brolin is no stranger to the action and comics movie scene, having starred in Grindhouse, Jonah Hex, and in the upcoming Sin City sequel, A Dame to Kill For.