I used to connect to PPPoE connections in Kubuntu with knet. Recently, I moved to Ubuntu to find out that Network Manager was unable to connect to DSL. However, it can be achieved using pppoeconf or knet, I wanted to use Network Manager. I figured out that it was a problem with the final release itself when I visited the forums (I had installed Ubuntu Karmic Koala Alpha 4 and then upgraded as new packages came in).
The newer version of Network Manager did not have this problem but it is still not available on the main repos at the time of writing this post. There is a ppa however, which helped me. It contained updated version of the package which is https://launchpad.net/~network-manager/+archive/trunk.