ifcfg-ippp0 contains DEFROUTE="no". After bringing up and connecting the device there is no default route. (At least with the fixed rawhide version). But as soon as you disconnect the ippp0 device (HUPTIMEOUT or manual hangup) there is a default route pointing to ippp0. Isn't a problem with ipppd so i filed this bug here. Also happens with latest rawhide code.
Closing bugs on older, no longer supported, releases. Apologies for any lack of response. If this persists on a current release, such as Fedora Core 4, please open a new bug.