I've just run netstat on my Slack machine and I see:
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 *:x11 *:* LISTEN 2298/X
Since everything else seems pretty nice and tight on a default Slack install, and since, googling around it seems the "-nolisten" argument can be used with X to stop it from listening for TCP connection - I'm a bit puzzled as to why the Slackware default install doesn't use "-nolisten".