It depends on whom you ask. A (very) strict reading of the core rules says that any enhancement bonus works for bypassing DR. The Sage and the MM2 say that it requires a magical enhancement bonus. Of course, the only Sage ruling I've seen about it was rather odd, also ruling that a natural enhancement bonus was sufficient to sunder magical weapons, and the MM2 entry contains obvious and amateurish rules errors, such as indicating that masterwork weapons have an enhancement bonus.
BTW, an adamantine greatsword has a natural +2 enhancement bonus, not a +2 natural enhancement bonus. There's no such thing as a bonus of the type "natural enhancement."