From reading and trying a few things, killing saslauthd and starting it using:
saslauthd -a pam -n 5 -V -c -m /var/spool/postfix/var/run/saslauthd -r -d
from the command line works. I'm guesing it's the -r that I need. not sure how to make this automatic, though.
OK, got it. In /etc/sysconfig/saslauthd I added -r to the flags line. Any drawbacks to doing this?
# Directory in which to place saslauthd's listening socket, pid file, and so
# on. This directory must already exist.
# Mechanism to use when checking passwords. Run "saslauthd -v" to get a list
# of which mechanism your installation was compiled with the ablity to use.
# Additional flags to pass to saslauthd on the command line. See saslauthd(8)
# for the list of accepted flags.