According to Cinema Blend on Aug. 27, Vin Diesel had revealed that he had been hyping the news about him being the voice of Groot in "Guardians of the Galaxy" for a more specific reason. Apparently, he had been meeting with Marvel Studios in regards to phase three of the Marvel Cinematic Universe.
Though most fans know that the only dialogue that Groot uses in the comic books is "I am Groot." and that's pretty much it, but his motives also pointed to future projects with Marvel Studios.
He didn't give specifics, but he had met and talked about something in regards to "IP" or intellectual property for something happening in the future around the years 2016 or 2017. Also, the pressures from fans had triggered the announcement for Diesel's Groot role. It sounds like his work with "The Iron Giant" film could be made comparable to see what he could do with motion capture for Marvel's role.
It seems that only speculation could be made as the actor was speaking in certain tenses and could be left up to interpretation, but fans my still wonder if Diesel were to have a role or not in the future as a live-action character.