In the process of installation ubuntu is trying to configure network via dhcp. For people using only a dsl modem without router that point is already a problem, something which urges them to look up the web, search the ubuntu forums they might not even know by that time and generally might keep them off using ubuntu as a desktop for their everyday tasks.
Would it be possible to integrate pppoeconf more neatly into the installation process, say if dhcp don't lead to any usable result? Would it furthermore be possible to add some pon/poff buttons to the taskbar in case a direct modem was detected while installing? I'm not sure about how networking is done in all of your countries, but here in germany there's lots of people with exactly that setting and they do not all use flatrates.
For those who understand German language:
vgl.
http://www.heise.de/newsticker/foren/go.shtml?read=1&msg_id=9946081&forum_id=93784
I suggest to add that functionality - and maybe the same thing for analog modems / ISDN to the installer.
I had very painful and difficult time trying to figure out how to set up my PPPoE connection. Fedora Core and Xandros make this very easy to do in a GUI manner. In fact, I think that the default GNOME Network Connections manager allows you to do this quit easily, but not in Ubuntu.