Of course, that had been the fan suspicion since the character appeared in the closing credits of Marvel's The Avengers last year--but what really changed the tone of the discussion and made rumor into accepted gospel was a comment made by Jim Starlin at a comic book convention last year, in which he told a fan the Mad Titan would appear in both of the big Marvel team-up movies.
It was widely assumed, especially after reports that Marvel had left Starlin (who created Thanos) out of the loop regarding the character's appearance in The Avengers and then been caught off-guard when the fan press expressed solidarity with the creator over the studio, that Starlin had been brought to Marvel and apprised of future plans for the character. While that might still be the case, Starlin has taken to his Facebook page to clarify what he does--and doesn't--know about the movies.
"Setting the record straight: It keeps getting reported that I claimed that Thanos will be in both the upcoming Guardians of the Universe [sic] and Avengers II movies," Starlin wrote. "Sort of true but not really. All I said, in some interview way back when, was that I had heard (as in convention scuttlebutt) that the Titan would be in both movies. I have absolutely nothing to do with Marvel films and not all that much with Marvel Comics. So I am no reliable source to quote on anything to do with what's happening in any future Marvel film."