Sorry if most of this has been said already, but...
I voted "Other", and here is my explanation!
"Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poison, a monk’s stunning, and injury type disease. Damage reduction does not negate touch attacks..."
It depends on the type of damage. Let's take a few examples...
1) A character has DR 10/magic. He gets hit with a claw attack that does poison damage and regular damage. It only does 9 points of damage, which means it doesn't actually do any damage at all, and the additional poison damage doesn't apply because its delivered through damaging (injuring) an opponent.
2) The same character gets hit with a Monk's stunning attack, which only does 9 damage, and so isn't stunned because he has to be damaged to be stunned (and is never actually damaged because his DR kicked in).
3) The same character gets hit with an incorporeal touch attack that does strength damage. He doesn't actually get hit, just touched, and the attack damages his strength. DR does not apply here because DR prevents you from being injured (damaged) unless you're being attacked with something that overcomes your DR - in this case, magic.
4) The same character gets hit by an opponent with Wraithstrike. It allows his opponent's regular attacks, which are physical swings of a sword or whatever, to bypass armor (which is basically what you're losing when your touch AC applies instead of your regular AC) and only require his opponent to "touch" him with his weapon in order to inflict damage. In this case, DR still applies because he is still being touched with a weapon. The same reasoning would apply if the weapon used with Wraithstrike was also poisoned - just because the weapon touches a creature with DR doesn't mean he's automatically poisoned. He is only poisoned if the attack still does enough damage to negate the damage reduction. Just being a touch attack is not enough.