Per this thread: http://simplicitypvp.net/forum/viewtopic.php?f=5&t=209 the nether will be reset when 1.9 (or whatever the next update is going to be called) is released.
If you have anything stored in the nether, be sure to move it over to the regular world before the nether is reset. I will not be moving things from the old nether over for you.