OK, "setenv LC_COLLATE POSIX" works. I'm not sure I want all sorting to be in ASCII order, though, just ls. I guess eventually I'll find out what other programs respect locale.

By the way, here's a footnote from the ls info page:

   (1) If you use a non-POSIX locale (e.g., by setting `LC_ALL' to
`en_US'), then `ls' may produce output that is sorted differently than
you're accustomed to.  In that case, set the `LC_ALL' environment
variable to `C'.

I guess the defaults changed or something, because I don't have `LC_ALL' set.