Guess what other Marvel villain will be making an apperance in The Avengers
Since we don't know yet how exactly it works into the story, I'm not entirely clear how big of a spoiler this is but considering Marvel is being very secretive about THE AVENGERS, we'll mark this as being a SPOILER and I'd advice you turn away if you don't want to know anymore about the film.
We know that Loki was reported to be the big baddie in AVENGERS, something that was hinted at in the post-THOR stinger as he tracks down the Cosmic Cube (which plays a large part in this summer's CAPTAIN AMERICA). Loki will use the Cube to contact the Skrulls (how exactly is unclear) and bring them to Earth to cause massive destruction. But Latino Review is reporting there is another villain that Marvel is trying to keep under wraps.
The LR boys (who first brought us the news on Loki) are reporting that Thanos, the Mad Titan, will be appearing in the film with the Infinity Gauntlet (also seen briefly in THOR and on display at last year's Comic-Con). Little is known about his appearance yet but considering he's far more powerful than Loki and the Skrulls, he may be the film's big BIG bad.
Thanos' first appearance was in "Iron Man #55" in the 70s but he didn't gain major prominence until a early-90s miniseries "The Infinity Gauntlet." The Gauntlet was a mystical glove that contained six gems that, when combined, granted the wearer mastery over Time, Space, Mind, Soul, Reality, and Power. What's interesting is that Thanos is not a particularly common villain for any of The Avengers. Regardless, it looks like he'll be making some kind of appearance (with no actor attached, it could be a CG-style creation).