Runner goes on ball 4

[rbmartin]

Yesterday in the Red Sox/Yankees game There was a runner on first and a 3 ball count on the batter. As pitch is released, runner on first attempts to steal. Pitch was called ball 4, but the catcher throws to second anyway. The baserunner slides into 2B safely before the throw arrives, touches the bag, but his foot comes off the bag due to momentum of the slide. The SS fields the throw and holds the glove on the baserunner throughout the sliding motion, including the portion of the slide when the runners foot briefly leaves the bag. The ump calls the runner out. The T.V. commentators thought it was the correct call and the Bosox Manager didn't seem too upset. Is this the correct call and what is the rule concerning this?