Although "winbind separator = \" is the default setting, it seems to be
impossible to set it explicitely. Here's my attempts:

smb.conf says:
   winbind separator = \
   winbind use default domain = no

 (note: the setting in the second line is irrelevant, but is shown because it
        gets dragged up onto the previous line by the "\")

testparm says:
   ERROR: the 'winbind separator' parameter must be a single character.
        winbind separator =    winbind use default domain = no

smb.conf: winbind separator = \\
testparm:
   ERROR: the 'winbind separator' parameter must be a single character.
        winbind separator = \   winbind use default domain = no

smb.conf: winbind separator = \\\
testparm:
   ERROR: the 'winbind separator' parameter must be a single character.
        winbind separator = \\   winbind use default domain = no

I also tried various things like adding a trailing space after the \, a blank
line following the \, etc. I can't get any of these things to work.

Why bother when this is the default setting? Well, I started chasing this up
while trying to find a workaround for bug #3238 - I though setting it explicitly
might help.