I remember seeing it somewhere, dont remember where though.
"I Was Out Of Name Ideas" found a link where it was mentioned but reading it closer it only says "But fear not! If you want to get into one of their areas without a fight just find a suit of their armor, slip it on and walk into the camp. Most of the enemies can't even recognize you through the disguise."
By disguise they mean wearing the armour common to that faction just like in FO2 when in the Enclave base. As long as you stay in the Advanced PA you will be fine. Take it off and you're died.
If it's not that one maybe it's another. Here's "I Was Out Of Name Ideas" post ==>
Here i just google it and found http://www.gamingexcellence.com/pc/games/2322/p20100617.shtml This isn't how I imagined it from what chris said but on the 2nd to last paragraph it brings up how the disguises work.