No one reads old SlashDot threads...
Yes, it's AX=ABS(AX).
I certainly wouldn't ding someone for not ripping though the answer - it's way too obscure. I've no problem explaining what each instruction does. But after that, it's pretty good at showing a thought process. Go get off my lawn, but dammit I think any geek should understand bits and bytes, and enjoy them. So even if they don't crank it out, I'd get a sense of the candidate, whether they thought the problem was pretty cool or a waste of time.
I once heard of some compiler geeks writing a program to find these code snips. "Gee, what five-or-fewer x86 instructions would perform x?" You supply a dozen inputs/outputs, the program tries all permutations of instructions, sees if In gives Out, Bob's yer uncle. The story (or legend) says the proggy shook out one or two new ones. If you don't think that's nifty, you're No Hire to me.