This has not been confirmed by Disney or Marvel at this time, and Josh Brolin's reps have no comment. The character was mentioned in a recent unnamed article about direct James Gunn and his post-production process on Guardians of the Galaxy. It states that Thanos appears on a rocket throne in Act 3, leading into the movie's big climax. Thanos will appear on-screen long enough that someone will have to step in and bring him to life.
Thanos first appeared in the Post-Credits scene from Marvel's The Avengers, where he was played Damion Poitier. So far, the character, one of Marvel's biggest and baddest villains, has not been teased in any of the official trailers, posters or images from Guardians of the Galaxy. Thanos is a member of The Eternals, a group of god-like beings with immense power. Often referred to as 'The Mad Titan', Thanos has tried to wipe out earth on many occasions, using his Infinity Gauntlet, which is capable of manipulating time, space, reality and the human soul.
Marvel has not revealed how big a role Thanos will play in the overall story arc of its Phase Two and Three movies. It is believed that The Collector, played by Benicio Del Toro in Guardians of the Galaxy, is on a mission to collect powerful mystical articles that make up the Marvel Cinematic Universe's version of the Infinity Gauntlet.
Expect Marvel and Disney to confirm this casting news in the very near future.
Guardians of the Galaxy comes to theaters August 1st, 2014 and stars Chris Pratt, Dave Bautista, Zoe Saldana, Michael Rooker, Ophelia Lovibond, Lee Pace, Glenn Close, Karen Gillan. The film is directed by James Gunn.