In Minecraft snapshot 12w36a (by mojang) a new variant of mobs has been added to the nether. I went to investigate and found myself skeletons and a new mob added called a wither skeleton, it has been mojangs secret for a while and in the next patch the wither skeleton will be added. The wither boss will be added soon hopefully, guessing why its in test mode currently is due to the fact it needs a habitat in which can be hostile to the wither, there is yet a place that suits the wither the most. My opinion is that the wither should originate in a stronghold and defend the portal. Let me know what you think and please watch my video.