In OS X pre-10.3, this was restricted by the driver. However, Panther appears to have changed this, although it isn't easy, or convenient, to set up. However this can be fixed. First copy the file /System -> Library -> PreferencePanes -> Network.prefPane -> Contents -> Resources -> SectionList.plist to a safe location. Next edit the file, and change this line:
"AirPort" = ( "AirPortSection", "TCPIPSection", "AppletalkSection",
"ProxySection" );
to read:
"AirPort" = ( "AirPortSection", "TCPIPSection", "PPPoESection",
"AppletalkSection", "ProxySection" );
i.e. insert "PPPoESection" into the list. NOTE: The above lines are shown on two rows for narrower display; enter the data on one line, not two, when editing the file!
(Re)launch System Preferences and you can now use PPPoE with AirPort using the PPPoE interface familiar from ethernet configuration.

