[edit: this was meant to be a reply to another_vulture but my error detection mechanism didn't spot that I pressed the wrong button]
" if they can provide NonStop on modern hardware. Sadly, this means Xeon."
"The NonStop architecture depends on fault identification features at the hardware level, some of which have only recently been added to the Xeons. "
Details welcome. Tandem kit hasn't needed custom CPU chips for a long time (since before the MIPS CPU days?).
As I understood it, the heavyweight synchronisation and comparison stuff in recent years was done in off-CPU logic called the Logical Synchronisation Unit. It's hard (maybe even silly) to even think about instruction level synchronisation when modern high performance chips are all cache based, and are consequently subject to soft (recoverable) cache errors and other trivia which would rapidly destroy any hope of instruction level (or memory access level) synchronisation.
Synchronisation when the OS does an IO operation (or something conceptually similar) remains entirely practical (but not exactly simple).
"sufficiently different from UNIX to make porting infeasible. It's not clear if this is also true of ... VMS."
Those familiar with the VMS sources and the relevant history (and who are not hampered by the alleged ongoing non-disclosure agreements) suggest there is no overwhelming technical reason why a port of VMS to AMD64/x86-64 would not be sensible. Commercial/political factors are a different question. There have even been suggestions that a skunkworks/proof of concept AMD64 port existed briefly inside DEC (covered by ongoing NDA).
Equally, since HP earlier this year finally admitted that IA64 was dead, there has been talk of reviving an x86-64 port of VMS, but outside HP (since there are some customers who are more interested in VMS than HP are willing to acknowledge). Will it fly? Who knows. If it doesn't, it won't be because technical reasons blocked it.
"This may be the reason that HP did not do this earlier."
Someone important in HP/Intel bet something big on the success of IA64, whereas what they should have bet on was the continuing importance to customers of their investment in *software* -be it HP-UX, NSK, or VMS. That HP/Intel person lost that bet, that was plainly apparent years ago, and customers have been paying the price ever since. Finally HP are being forced to admit their stupidity.