I can't say if this sort of phenomenon has afflicted 911s but in the BMW community the alusil blocks are the ones sought-after and the nikasil blocks are considered unreliable and defective.
This is because, as BMW found out in the mid-90s, the nikel in the Nikasil V8 engines reacted with the high sulfur content in certain American city gasolines, eroded the blocks, and caused compression failure in short order. In response BMW extended its powertrain warranty for Nikasil engines in the mid 90s to 6 years/100,000 miles and used Alusil blocks as a replacement fix for the bad engines. For more details see
http://www.koalamotorsport.com/tech/misc/v8shortblock.htm
BMW did not have this problem with its Nikasil motorcycles and perhaps, like them, the 911 engines are secure. It would be interesting to hear anyone who has some additional input on this.