With FC1 you don't have to have eth0 up. In fact, it's suggested that you do not bring it up nor assign it an IP address. I came across postings stating that you may need an IP address, but in my case I don't.
Perhaps 'iface=ppp0="up" ?
In my case, the config file for pppoe is /etc/sysconfig/network-scripts/ifcfg-ppp0
It can also be /etc/pppoe.conf on some distros
Perhaps this page will help:
http://www.linuxathome.net/pppoe.php (even though it's about an rpm based system).
Good luck. Perhaps LQ will add a Gentoo forum in the future.