Originally Posted by pompousjohn

This happened to me recently as well.

Here is what the tech told me was the issue and how he fixed it:

When the thermal grease on the processor gets hot repeatedly the thermal grease breaks down and no longer does it's job, fixing it is as removing the old thermal grease and replacing it.

Unfortunately, the thermal grease available off the shelf is not the same grade Sony uses, so even if it is repaired the chances of it YLOD'ing again are pretty high.

Sure enough mine was repaired with a 30 day warrantee, and YLOD'ed again around the 45 day mark.