While Marvel Studios has still yet to officially announce that Vin Diesel has been cast as Groot in Guardians Of The Galaxy, the actor continues to tease the role to his Facebook fans. Earlier today, Vin Diesel posted a teaser photo of himself in front of a microphone with an image of Groot in the background.
The image of Vin Diesel doing voice work is actually from his time working on the Iron Giant, and the Groot image was just photo-shopped into the background. It's unknown if Vin Diesel has actually done any of the voice work yet for Groot. If the movie follows the comic books, then Vin Diesel will only be saying "I am Groot," which means it probably wouldn't take him very long to complete his work for the role.
The Guardians Of The Galaxy is scheduled to be released in movie theaters on August 1, 2014.