Less than 1%. It has a very low rate of occurrence, and is easily fixed without the help of Sony on the off chance it does happen.
@jalaneme
12,000 out of 46,600,000 (46.6 million PS3 sales to date) is still less than 1%. You would need 466,000 YLOD's at this point in time for that to be a 1% failure rate.
In short, 12,000 is barely anything.