I've heard of two different causes for the YLoD. The first, and most common, is basically the same thing as the typical RRoD. Heat causes the motherboard to warp slightly, which places strain on the solder contacts. This causes damage which accumulates over time, until the contacts break and the system dies. The second cause is power supply failure. Realistically this can only be fixed by replacing the power supply, so it's something only Sony can do. Check out the thread on the PlayStation forums:
http://boardsus.playstation.com/playstation/board/message?board.id=ps3&message.id=3629439
This problem is far more widespread than most people think. If you do send your system in for repair, it is possible that it will break again - and Sony will charge you for repairs every time it happens. So far the defect seems to be limited to launch systems, but it's still too early to be sure.
