The default install script looks for /var/mysql/mysql.sock, which is not where OS X 10.4.11 and MySQL installer for 4.1.18 were installing the socket. To get it to work, I had to:
sudo mkdir /var/mysql
sudo ln -s /tmp/mysql.sock /var/mysql/mysql.sock
..in order to proceed. I'm also filing this as a code bug, I suggest both teams hit it (with a sledgehammer), so that install *and* install docs cover this.