From the Magic Items section of the DMG:
"Any item with intelligence has an alignment."
From the Monster Manual:
"A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature."
So, if a creature with the Evil subtype overcomes DR X/Evil as if his weapons had an alignment that match 'Evil'...
... and if we accept that his weapons will, in fact, overcome DR X/Evil...
... then a weapon with an alignment that matches 'Evil' will overcome DR X/Evil.
And an intelligent item has an alignment.
So if that alignment matches 'Evil', it could be argued that it will beat DR X/Evil... even if it's not, in fact, an Unholy weapon.
(I'm saying it
could be. Not necessarily that I'd use that argument myself

)
-Hyp.