Though his reps vehemently denied this (and an accompanying rumor that he walked out on a script read)...that was yesterday.
Today...every TV news source out there, including the always reliable Michael Ausiello, has stated that Knight has in fact asked to be released from his Grey's contract.
Furthermore, the Grey's producers are expected to oblige. It's currently unknown how he will depart the program.
And will the show take a hit without George or is Grey's quickly morphing into the next ER (as in major characters come and go and it doesn't really matter)?