In addition, Ant-Man, Thor 2, and Iron Man 3 are part of the list (with Thor and IM as definites).
Via io9, here’s the info on Guardians of the Galaxy and the Inhumans:
Guardians of the Galaxy: Set in an alternate future, this superhero team protects the galaxy from all sorts of alien invaders. This newly rebooted pack of heroes is made up of members who are the last of their kind, including a crystalline being from Pluto and a soldier from Jupiter. Feige says “There’s an opportunity to do a big space epic, which Thor sort of hints at, in the cosmic side of the [Marvel] universe.” (Editor’s Note: For more on “Thor 2” being cosmic check out this and this)
The Inhumans: Millions of years ago the alien species Kree started experimenting on early humans looking to build a race of super soldiers they could use to fight The Skrulls (the rumored alien baddies from the Avengers movie). After the aliens abandoned their human test subjects, the mutants eventually formed their own society, and are now what Marvel calls The Inhumans. If either this one or Guardians of the Galaxy becomes a film, Feige says you should expect an X-Men style ensemble, along the lines of The Avengers.
Ant-Man: EW says it’s “still nebulous but definitely in the works.” Edgar Wright has “developed an excellent draft recently,” says Marvel’s President of Production, Kevin Feige. Wright has been working on said script with the director of Attack The Block, Joe Cornish, for months and months.
The Avengers assembles May 4th, 2012 and is directed by Joss Whedon, starring Robert Downey Jr. as Iron Man, Chris Hemsworth as Thor, Chris Evans as Captain America, Samuel L. Jackson as Nick Fury, Mark Ruffalo as the Hulk, Scarlett Johansson as the Black Widow and Jeremy Renner as Hawkeye.