He dived. He simply chose to fall, way after the actual foul was made. Since it's very hard for the referee to see how serious such a foul was, it all depends on the body language of the fouled player. It was an obvious penalty foul seen from the replays (Cahill being dribbled, puts both hands on Mirko to stop him) but with Mirko on his feet during and shortly after the foul, he gave an impression that it wasn't even a foul. When he finally decided to fall, it was obvious that he's diving. If Mirko fell when he should have, it would have probably been a (deserved) penalty.
