His chin is good, it's not great and not horrible. What is really impressive about him that even when he's hurt or stunned sometimes, he tries to hide it SO well that his opponent doesn't even know, or when he's spotted hurt he knows how to react/hold/go defensive mode till he recovers.
People need to forget about what happened to Cotto at 140, it has nothing to do with the Cotto of today at 147. Cotto has a solid, but definitely dentable chin. He also has great heart and is becoming more and more relaxed in the ring, which benefits his ability to take punches in general. He's lucky that the B+/higher fighters in the division don't have real KO power.
Torres hurt Cotto really bad in that fight. ALOT of refs would have stopped it. Cotto has a decent chin but its not a damn good rock hard chin. What he does have is alot of poise and heart. He gets hurt but keeps his cool.
His chin isn't great, but it's solid nonetheless... plus his recovery abilities and resilience are excellent. He's a bit like Frazier was at heavyweight in that regard: not unlimited durability, but it takes a great puncher to stop him. We'll have to see whether Margarito is Cotto's Foreman, but i think Cotto takes whatever punch Floyd has just like Frazier took everyone Ali had.
Cotto and a few other guys get unjustly tagged with the glass chin label. If getting hurt, getting knocked down, or even getting knocked out meant that a fighter had a poor chin we would have to conclude that Alexis Arguello Danny Lopez Rocky Marciano Joe Frazier Roberto Duran And countless other greats all had china chins. Only a fool would make such a claim.