Updated August 1: If you haven't seen Guardians of the Galaxy yet... via a Marvel Phase 1 & 2: A Look Back sizzle reel (below), here's the first image of Josh Brolin as Thanos in Guardians.
As revealed at Comic-Con last Saturday the actor is set to reprise the role, presumably for the eventual Avengers 3. At the panel, Kevin Feige setup Robert Downey, Jr. by saying, "I don't know how you top Ultron villain-wise!"
Downey responded, "Marvel always has the next big thing!" and Brolin walked out on stage wearing the Infinity Gauntlet, Thanos's weapon of choice that allows him to wield all six Infinity Stones - 3 of which have been introduced into the Marvel Cinematic Universe.
Original story:Rumored as far back as April, today Variety and Latino Review are reporting that Josh Brolin will be playing Thanos, the Mad Titan, in Guardians of the Galaxy. There is no confirmation directly from Marvel Studios yet. While the role is suspected to still be more of an "overlord" or background player in that film, with Ronan the Accuser and his lieutenants Nebula and Korath playing the main active villains, this is likely casting intended to carry the villain through to Avengers 3, long thought to cap off the story of Thanos teased in the stinger to The Avengers, and furthered by the reference of the Tesseract (Cosmic Cube to comic book fans) and Aether as "Infinity Stones" in the stinger scene to Thor: The Dark World.
Thanos's involvement in Guardians of the Galaxy was all-but-assured from early in the process. There's the familial connection of Nebula and Gamora to the character, of course, and the in-continuity prequel comics that Marvel Comics has been publishing for the film have featured Thanos heavily, giving the character lines and direct involvement in the events leading up to the film.
Those comics have also liberally teased that the orb seen in the very first preview images and trailer, the one Star-Lord is attempting to steal, is in fact the third of the Infinity Stones. These stones, known mainly as Infinity Gems in the comics, each represent a different primal force: Soul, Power, Time, Space, Mind, and Reality. Individually, each is extremely powerful, but combined, and usually worn on the Infinity Gauntlet (of which Thanos is the primary wielder), they make the holder a godlike entity. The gauntlet? Well that's already appeared in the Marvel Cinematic Universe, too, as an easter egg in the armory of Thor's Asgard.
Variety notes that Brolin's Thanos is also likely to appear in Avengers: Age of Ultron, at least in a cameo. With no confirmation, there's also no word on whether the actor will simply provide his voice, or will (more likely) follow suit with James Spader, who is providing motion capture as well as the voice of the robotic Ultron.