It took forever, but I got PPPoE on a WiFi connection working on my n900. Roughly here are the steps. I will make a better walkthrough when I get the time
1. Install the power kernel with pppoe support
2. Install ppp
3. Edit the ppp config files to add your creds, use pppoe, and dns
http://www.faqs.org/docs/Linux-HOWTO...TO.html#MANUAL
4. write a script to configure wlan0 up and associated with the appropriate network
5. write a script that calls pppd with the correct username (you need to specify it both in the config files and on the command line)
6. write a script that clobbers /etc/resolv.conf with /etc/ppp/resolv.conf (note this prob isn't the right way to do this because the N900 uses dnsmasq but I haven't figured out how to do it the right way)
7. install the libicd-network-null as detailed at the bottom of this link http://wiki.maemo.org/N900_USB_networking
You need to do this for the maemo applications to play nice with the interface