In reply to flibble:
Windows - does it still run on those? If I bought a windows CD from PCw*rld could I install it on any of those, and then install, say, Office on any of those?
Macs - I mentioned that - you should read more carefully. The point here is that they are *transitioning*, ie moving from one to the other, not broadening their architectures covered. This is critical because it is the crux of my argument.
Linux - mentioned again. An exception. For the reason why, see the previous post.
BSDs - OK, another Linux-like OS. Not a commodity OS.
Solaris - a UNIX, not a commodity OS
AIX whats that? (A unix IIRC) I assume not a commodity OS
HP-UX - hmmm, isn't that a UNIX variant? </sarcasm> Not a commodity OS.
Amiga OS. Don't know much about that, perhaps that is transitioning too.
I also don't think you can really count itanium in there as a separate architecture from x86, for a couple of reasons:
1) It runs x86 code. While chip people quite reasonably consider different internal structures as different architectures, from the OS vendors view supporting something which works already, however slowly, is an easy tick for the back of the box.
2) It was expected to be a replacement for the old cores, then bombed. I'm not suprised various OS vendors looked at supporting it, as a *transition* from the old-style x86.
Also if you claim that I can claim RISC OS is already multi-architecture because it can run on many different types of ARM.
I mentioned the embedded market, briefly as well. That is interesting in that it has the same or similar OSes on many hardware variants (although often the same processors), and yet they are not compatable from the consumers point of view. Nevertheless, I guess these count. Why don't you get them for desktops though? Maybe they've switched one set of constraints for another.
I think what is clear is that the 'big' OSes end-users use currently have to align all their (current, actively purchasing) users into one binary-compatible group. Transitions, like Apple did recently, may be painful, but less painful than trying to maintain two or more separate groups on different chips.