For those that do not know about this, some factions when vilified with them WON'T attack you.
I've noticed this with the Powder Gangers, NCR and the Legion so far. And both the PC and Xbox 360 version has the problem, not too sure about the PS3.
I know many people have this problem and to be honest it is game breaking, no matter what people say. How can you possibly play a game where your enemies will not attack you first?
Before people start trying to be all smart let me say I am not wearing faction Armour.
So please Bethesda, fix this mistake you have made. It's been going on for far too long, your customers would like to play the game they bought which you made.