"a HP Superdome with x86 processors"
"if you consider the similarity from a chipset perspective between Westmere and Tukwila processors now, it's not that difficult to imagine a HP Superdome with x86 processors either..."
Everybody with a clue knows that, but be careful where you say it. Round here, you'll wake one of the few surviving IA64 boosters, and he'll accuse you of being an ignorant troll. He won't expand on why it's a silly idea, obviously.
"The biggest challenge as you extend to x86 systems of this size isn't the resiliency of the hardware, it's how the OS interacts with the hardware and firmware..."
Again, everybody with a clue knows that...
"during failure conditions "
Not entirely sure that qualifier needed adding, but I do see where you're coming from. You can't easily swap out a whole Superdome at a time, and there'd probably be some reluctance to swap major parts of a high end Proliant without good reason...
"Linux and Windows sadly lag behind the commercial UNIX OSs at the moment"
Again, yes indeed, with little sign that the problem is even understood, let alone any sign of it being addressed.
"how well Solaris/x86 handles all this, but my bet is poorly given their general poor showing in this space even on SPARC"
Don't know Solaris/x86 myself but when AMD64 came out, one of the Solaris chaps (whose name I sadly can't remember) used to blog about some of the new things AMD64 gave them from a RAS point of view, and it read nicely. What it turned into in the shipping software, I have no idea.
Summary: There's more to "mainframe-class RAS (reliability availability and serviceability)" than just The Chip Inside(tm).