Filming for Guardians of the Galaxy is currently underway, but there are still some characters that have not been cast. Those would be Groot and Rocket Raccoon. Vin Diesel is apparently on board to voice Groot but it hasn't been officially put through yet, but the rumor now is that Bradley Cooper (Silver Linings Playbook) has had the offer to voice Rocket Raccoon extended to him.
James Gunn has expressed his love for Rocket Raccoon and has stated a big factor in him even directing Guardians of the Galaxy was because of that character. However, this is a rumor about an offer so if it's true at all it is possible that Bradley Cooper might not accept the role. There is some possibility as Cooper does have some down time as the filming for American Sniper has been delayed after Steven Spielberg dropped out, but there is no guarantee. We will just have to wait for an official press release on this one. Marvel is pretty secretive about their casting news until they want people to know.