I think it was Latino Review that first said he was in negotiations – having previously said Marvel wanted Adam Sandler or Jim Carrey, which they may well have at one time. Now Marvel has confirmed he’s signed and sealed. Vin Diesel is still not 100% locked down as Groot, but everyone assumes he will be.
My reaction to this is more in the Heath Ledger/Michael Keaton ballpark: he’s not who I pictured, but I know he can act. Ladies out there with Furry fantasies, you may rejoice now.