Overcoming Damage Reduction with a Sneak Attack

3.5 Edition Rules:

Many situations were not clearly addressed in the 3.5 edition of books.  One case is how to treat sneak attack damage when attacking an opponent with Damage Reduction (DR).  Fortunately for us, this case is specifically addressed on the Wizards site in one of the excellent "All about..." series.  In "All About Sneak Attacks (Part Three)", Skip Williams tells us that the damage from a sneak attack does count toward the amount needed to overcome DR.  Therefore, if your rogue deals 4 points of weapon damage against a creature with DR5, and includes 7 more points of damage from the sneak attack, the total damage of 11HP is enough to overcome the DR5 and the creature will take (11-5)=6 points of damage from the attack.

Source:  http://www.wizards.com/default.asp?x=dnd/rg/20040302a