Michael Denny, vice president of Sony Worldwide Studios Europe, has spoken to Silicon Republic about the prospects of PlayStation 3 in the light of the PS4 announcement and interestingly he mentioned Gran Turismo 6 as a PS3 release:
"Look at the games coming out on the PlayStation 3 like The Last of Us, Beyond: Two Souls, GT6, and then on the third-party side of things another fantastic Assassin's Creed, GTA. It's important for our content to have that kind of longevity because, as I was saying [about the PS Vita], the games that come through are going to get better and better over time."
Of course, this doesn't rule that GT6 could make it to PS4 as well, but everyone had just sort of assumed it had been shifted over to PS4 already.