For a PPPoE Passthrough on DD-WRT, enable jffs and do the following steps on a *nix-based system
cd /tmp; wget http://kamikaze.openwrt.org/8.09.1/brcm-2.4/packages/rp-pppoe-relay_3.10-1_mipsel.ipk; tar zxf rp-pppoe-relay_3.10-1_mipsel.ipk; tar zxf data.tar.gz; scp usr/sbin/pppoe-relay root@:/tmp/
Start the relay agent on the router
/tmp/pppoe-relay -S vlan1 -C br0
Now you can start your pppoe client on your notebook.
Tested with a WRT54G and DD-WRT v23 !