Ok, minor crisis... Seems that DigiTunnel as of today does not support Panther. Using the "Internet Connect" application, I get disconnected immediately, and I just realized that OSX' built-in PPTP client uses CHAP authentication.
My Internet connection depends on a PPTP connection with a PAP authentication, and it is out of my hands to change this, so up till now I've been using DigiTunnel with great success. However, they have yet to release a Panther compatible version, so until they do, is there another way to force a PPTP connection to use PAP authentication?