Although the username should not be secret in most cases, it contains confidential information at some providers. E.g. the german T-Online builds the username from the phone-number and a secret, long number (which must be entered in T-Online's own-application into a veiled password-input field). So it's not good when giving this username as a cmdline-option to ipppd in /etc/init.d/isdn, because everybody can see it with `ps'. Options can be given to ipppd in a configuration file with '-f...', so the username can be put there.
i have fixed this problem. I will build new version internet-config-0.23 today or tomorrow. Note: in 7.1 beta3 is internet-config a replacement of isdn-config.