bluray readers are usually the culprit I personally know of 3 people on my friends list that have had that problem but I think they leave the game on and pause and leave over night or something.
I know one of them did that for sure my nephews was still under warranty but his mom bought a extended warranty from target and lost the reciept he never got it fixed.
now he got a 360 for COD2 mostly because thats what me my son and his brother play on we all have ps3 too except my son we share.