AMD/Intel CPU Yield/Failure Rate
Yield rates are definitely a commercial secret; they will likely vary from batch to batch with normal manufacturing variation and attempts to tune the process to increase yield.

Yield is inversely proportional to die size. The i7 die size for "Lynnfield" is 296 mm², according to wikipedia, which is pretty big.

Yield is also traditionally low on newer manufacturing processes. Intel are always on the cutting edge, as part of their "high performance / high cost" market strategy.

Spot failures are usually due to tiny imperfections in the silicon substrate crystal. There are also the usual alignment and patterning issues which may cause whole wafers to fail. Generally the whole thing is a very nasty process control problem; dopants and chemical reactions have to be applied completely evenly, again and again in each of the layers. A single tiny bubble will cause a failure.