I actually just returned my CPU (Phenom II X4) to AMD, and they've replaced it, but they didn't say exactly why it died. I've asked them for more details, hopefully they can tell me.
Overall though, given with how many computers I've worked with, CPU failures still seem rarer than Memory, Disk, Mobo, or Graphics failures. Of course it ends up being the CPU in _my_ computer that fails -.-
There is still some variance in silicon, so yours may have had a defect that manifested itself after some time, I'm not sure they evaluate returned defective chips to see what happened (and if this is public info)
Also, the packaging is extremely complex and prone to the same kind of defects as other PCBs in the system.
Overall though, given with how many computers I've worked with, CPU failures still seem rarer than Memory, Disk, Mobo, or Graphics failures. Of course it ends up being the CPU in _my_ computer that fails -.-