From the SRD;
"If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation."
From this I would take that unless the DR type specifically states it stacks (like with a Barbarian's DR, or Fey's Heritage feat's DR), then they do not stack.
"If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation."
From this I would take that unless the DR type specifically states it stacks (like with a Barbarian's DR, or Fey's Heritage feat's DR), then they do not stack.