According to a Pathfinder designer, untyped damage overcomes all DR. The comments are recorded in the last entry in this d20pfsrd FAQ section (just before "Blink and Mirror Image (4/22/10)") and comes from this forum post.
Spells and effects that do untyped damage are pretty rare in Pathfinder, since these spells are quite powerful since their damage can't be stopped by any form of immunity, resistance, or damage reduction. — James Jacobs
This is surprising and counter-intuitive to me. As I understand it, DR 5/X is meant to mean "DR 5 bypassed by X", and I'd interpret untyped damage as simply never counting as any value of X, not something that magically ignores it altogether. The Overcoming DR section doesn't mention anything about untyped damage.
I'd like to fact-check this, since it's not unheard of that designers are wrong or confused about their games. By the rules we have actually written: does untyped damage really bypass all DR, even DR/—? Does it only bypass it if it's from a spell, or does all untyped damage from any source (e.g. falling damage) bypass DR? If applicable, please cite relevant rules in your answer.