Launch ppp at boot :

With Debian...

Edit /etc/network/interfaces :

auto ppp0 iface ppp0 inet ppp provider dsl-provider

dsl-provider living in /etc/ppp/peers

With Ubuntu, I only had to do this :

ln -s /etc/ppp/pppoe_on_boot /etc/ppp/ppp_on_boot and didn't modify /etc/network/interfaces as above... since Ubuntu uses Debian's deprecated method.