[or… how I learned to stop worrying and love the /etc/group file]

On Red Hat, the default permissions for a serial port are:

ls -l /dev/ttyS1
crw-rw----    1 root     uucp

This means that root is the owner and group is uucp. No others have access to the serial port. In order for a non-root user to use the port, then, you need to add the user to the uucp group. To do this, simply edit the /etc/group file. Find the uucp entry, and add a comma and the username you want to belong to the group:

uucp:x:14:uucp,usernametohaveaccess

In this example, usernametohaveaccess now has access to ttyS1.