Brett:
for user in $(cat usernames); do usermod -p $(mkpasswd $user aa) $user done
awwwww... how boring, all salted with the same salt, that's no fun :p
But nicely formatted, not using deprecated syntax (although that ought to say $(<usernames) rather than $(cat usernames), thinking about it), no gratuitous perl and it doesn't matter about the salt as the passwords are bleedin' dangerously obvious anyway. It's not too hard to format $RANDOM for the salt if you really want to, anyway.