I've asked Gnacik, the plugin author, if he's willing to implement nether islands that map to overworld islands. You can keep track of it in the comments here: http://dev.bukkit.org/server-mods/islandworld

You can comment on the question yourself if you like, but please be polite. The author has more then enough to do already maintaining the plugin and answering everybody's questions

If the author sais no, then that's that. No nether islands. Dom isn't going to spend his precious free time on coding another plugin. When he manages to finish the plugin he's working on now, he'll have more then enough work maintaining it and keeping it up to date as far as coding is concerned.