> x86 still takes approximately
>0% of the mission critical computing market

This is some US legacy thing. From my experience in Poland it's x86/x64 that make huge part of mission critical market, and in fact most of RISC/mainframe market is being slowly replaced by them.

The difference between "normal" systems and mission critical systems is mostly in configuration and storage:
1) mission critical ones use advanced clustered configurations, with often two geographically distant locations.
2) Mission critical ones always use large, FC-connected disk arrays (EMC, IBM etc.), often with mirroring between locations. It's actually the arrays and disks that make most of the hardware costs.
3) Most mission critical computer components are redundant and hotpluggable.

I've seen little interest in increasing CPU/memory level RAS beyond basics. For example most companies or governement institutions buy servers with memory mirroring but nobody enables it. And noone is interested in buying ECC enabled PCs.