I imagine that this will be one of the various content updates they'll add to online lobbies, as they mentioned (Not specifically) in their list of things they want to improve with the game.
Also wouldn't be surprised if they left that out completely, since most people prefer to chat via private PSN parties.
Plus if I recall correctly, lobbies in GT6 that had their voice chat disabled would tend to perform a little better than ones with it enabled. Considering the current state that the multiplayer lobbies are in, they will want to do anything they can to improve server performance.