I'll post some detailed info about Thanos at the end of the article just in case you want to avoid spoilers.
"Guardians of the Galaxy" has an August 1, 2014 release.
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Thanos Guardians of the Galaxy movie details:
James Gunn and Joss Whedon cast the actor (rumored to be Josh Brolin)
Thanos is said to appear as a hologram in the first act of the movie when talking to his minions (probably Ronan and Nebula)
Ronan is seen kneeling in front of Thanos, with only a glimpse of the Mad Titan shown
Thanos has both Ronan and The Collector go after the Infinity Stone
We do know one other thing, but we're not spoiling it (sorry!)
The Avengers Thanos Concept Art: