He's been out rehabilitating since his loss to Rosol in the second round of Wimbledon man, it's been almost half a year. He is 100% fit, but jus not a 100% match-fit, my thoughts are he doesn't want to give-away any psychological edge to his rivals going into the Clay season, he wants to hit the ground running and surge on from there. I personally think it's the sissy boy's approach, he should compete in the Aussie Open and give it his best go.
He is fit, he's jus not match-fit. Call me a cynic but I believe the "stomach bug" was a cover story and he and uncle Toni watched Novak today against Roadrunner and completely demolish him, so sort it best as he jus pulled out of the Aussie open. Chances were he'd meet Novak in the semi finals and it'd have been a huge psychological edge for him going into the clay season. I know I am going deep here, but this really does happen.
A true fan is one that indulges himself within the game - not the player's that play the game.