This seems like it would be a problem (imagine the full plate wearing barbarian) but I haven't read anything yet that suggest it can't be done (guy puts holes in armor for spikes to poke through maybe?).
Fiend Totem (Su)
Prerequisite: Barbarian 6, lesser fiend totem rage power
Benefit: While raging, the barbarian sprouts dozens of wicked barbs from her body. Anyone striking the barbarian with a melee weapon, an unarmed strike, or a natural weapon takes 1d6 points of piercing damage.
Do totem spikes work while wearing armor?
Yes, they work in armor. Nothing in the description says it doesn't. The descriptive text "the barbarian sprouts dozens of wicked barbs from her body" is not important--the key is the rules text: "Anyone striking the barbarian with a melee weapon, an unarmed strike, or a natural weapon takes 1d6 points of piercing damage."