Also, it was posted in the 3.5 thread:
" 5) The natural attacks AND weapon attacks of a creature with an alignment subtype (Chaotic, Evil, Good, Lawful) can overcome damage reduction as if those attacks and weapons had an alignment (or alignments) matching the creature's subtype(s). Fiends are really good at hurting celestials, and vice-versa."
This implies it's DR X/evil (or X/unholy, depending on how you view it) for Celestials. It's interesting that it extends to weapons they use now.
As for demon vs. devil, it's also been said that demons will tend to have DR X/cold iron and devils will have DR X/silver, which leads to the famous DR 15/Holy Silver that Pit Fiends have. Which, of course, brings up the eternal question: how can the Blood War work when neither side can beat each others' DR? There must be a LOT of silver floating around down there.