On SMP, with the world type set to ATG, nether biomes don't generate, despite "Add Biomes To Default World" being set to true.

It works perfectly in SSP though.

Tested on vanilla Forge and MCPC+.

Was about to proposition myself to have your babies, but I've just tested this on my server (using MCPC+), and a fresh nether still generates as a vanilla one.

Just tested it on a local vanilla Forge server and it's not working on it either.