Ask, and you shall receive. From the FAQ:
Q: A rogue in my party has no magic weapons but did a sneak attack against a monster with damage reduction 20/+2. How should this attack be resolved? Do I roll the damage for the hit, add the bonus damage from the sneak attack, then compare the total to the DR? Or is there no sneak attack damage unless the basic attack beats the DR? According to what I could find in the DUNGEON MASTER's Guide , special effects from ranged or melee attacks don’t apply unless they’re magical, such as fire damage from the flaming weapon, or the attack that delivered the special
effect beats the DR. So, is sneak attack a special effect?
A: A sneak attack provides bonus damage, not a special effect. In a sneak attack, roll the bonus damage and apply that against the DR. For example, a rogue who hits a foe that has DR 20/+2 with a nonmagic short sword for 6 points of damage doesn’t get through the DR. If the same rogue sneak attacks for 25 points of damage, 5 points get through DR.
If the same rogue struck the same foe with a poisoned short sword for 6 points of damage, the foe would not be damaged, and the poison would not take effect because the DR stops all the damage. The same rogue sneak attacking the same foe with a poisoned short sword for 25 points of damage would deal 5 points to the foe and the foe would then have to save against the poison.