"NEW ARMOR SPECIAL ABILITY:
TWILIGHT
This armor special ability, which first appeared in the Book of
Exalted Deeds supplement, is of particular use to duskblades.
Such armor becomes translucent when donned and possesses
a faint sunset-colored sheen. Twilight armor reduces the chance
of arcane spell failure by 10%.
Faint abjuration; CL 5th; Craft Magic Arms and Armor; Price
+1 bonus."
Page 22 of the PHB2. Crap, it's magical, but it is cheap at a cost of 5100 gp for a +5 AC and no arcane spell failure or armor check penalty.
