Seriously? SRIRIOUSLY!?! A feind is a evil person or animal. Every time its brought up it most likely refer to dangerous enenies in general.
One of the previews mentions a fiend camp, I don't know about you but if that's supposed to refer to bandits or slavers than why not just call it a bandit/slaver camp. Your interpretation of the word fiend here would be just as stupid as calling a slaver camp a boogeymen camp or a cad camp.
Fiend probably means some sort of gang, but I'm hoping it's a new enemy.
EDIT: it's from the second blog about the FNV http://bethblog.com/index.php/2010/09/21/fallout-new-vegas-official-strategy-guide-blog-crafty-beggars-living-off-the-land-and-on-the-lam/, here's the quote:
Or the most potent poisons with which to coat your melee weapon, and charge head-long into that Fiend encampment you’ve been wanting to ransack