At the risk of coming off a little more contrary than I intend: seriously, who cares? joezydeco was talking about teaching an assembly language with "training wheels." 8086 assembly without any doodads works for that. It's useless for normal people but no more useless than 6502, and the student will have development tools that are actually friendly to play with.
The nice thing about learning x86 assembly is that embodies the entire learning curve. If you want to inflict all the horrors of assembly language on yourself - a pointless pursuit for anyone not writing a compiler while simultaneously eating paste, but hey, whatever - x86 lets you start small and work your way up.
I'm just not clear on who needs elegant assembly language programming (outside of compiler writers, etc.). Doing it at all represents failure. The point of an assembly language class is to be very quick and make the student appreciate what C does for them, IMO.
Fair enough, you can certainly go your entire career without resorting to assembly language (heck, the last time I got paid for programming in assembly was the early to mid 90s---I'm still amused by the "C is too low level for programming" arguments these days, when back in the early 90s, people were bitching about C being too high level and inefficient).
I just don't know if it's wrong to jump in the deep end with x86. Obviously the argument can be made either way, and x86 is ugly, but I'm pretty prejudiced toward learning something that will potentially be useful right off the bat. That would make me lean towards x86 or ARM for teaching or for self study, I think, and I have no idea what the ARM tools look like right now.
One of my more masochistic long-term goals is to make a compiler (I've made an assembler of sorts) so I might be revisiting this question at some point. Joy.