The natural weapons of a creature possessing DR X/magic bypass DR X/magic, but many polymorph effects can't grant DR...
According to the Monster Manual errata the special ability DR X/magic is a supernatural ability, and supernatural abilities aren't gained upon using the 4th-level Sor/Wiz spell polymorph [trans] (PH 263) et al. The Monster Manual on Damage Reduction says that
Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters. Such creatures’ natural weapons (but not their attacks with weapons) are treated as magic weapons for the purpose of overcoming damage reduction. For example, the gargoyle has damage reduction 10/magic and can strike as a magic weapon for the purpose of overcoming damage reduction. (307)
Emphasis mine. (And, yes, really, the gargoyle is the actual example.) This typically means a creature's ability to bypass DR X/magic is contingent upon the creature first having DR X/magic, which, as I said, the spell polymorph doesn't provide. (Note that a specific creature's ability to bypass DR X/magic can override this general rule, but see below.)
...Although there are ways around this...
The 9th-level Sor/Wiz spell shapechange [trans] (PH 277-8) or the feat Assume Supernatural Ability (Savage Species 30-1) can grant a user one or more of a creature's supernatural abilities, including damage reduction.
(I'm not a psionics guy, but the powers metamorphosis and greater metamorphosis seem similar enough to polymorph and shapechange, respectively, that similar rules apply. A contributor with more expertise in that arena can address whether there's a psionics trick that makes assuming a supernatural ability available earlier or more easily than the feat Metamorphic Transfer (XPH 48).)
...And there are a lot of monsters
It wouldn't surprise me if there were a creature that possesses as an extraordinary natural attack the ability to bypass DR X/magic with its natural weapons. A creature that possesses such an ability would grant it to a caster who used polymorph to assume that creature's form. However, I am unaware of a creature that has such a specific extraordinary special attack.