hamilcarr & BTS,
If we identify the emergence of Jewish monotheism stricto sensu with Deutero-Isaiah, then it strikes me that it presupposes typical Persian dualism but attempts at surpassing it (cf. Isaiah 45:7). In a sense, it is born from this very attempt.
But this "pure," non-dualistic (or, better perhaps, meta-dualistic) monotheism, is essentially tautological: whatever happens, "good" or "bad," God does. Not very functional as a historical religion. Not suitable for political, or even ethical, purposes. Whence the historical recession into dualism.
From another perspective, the monotheism of Deutero-Isaiah also builds on a literally a-theistic strategy, which was bound to hit back in the long run: the (other) gods do not exist. From this perspective, too, incipient monotheism has at least as much in common with a-theism (the negation of gods) as it has with poly-theism (making one god the only one, thereby mutating him into more than a god -- God).
On edit: I put a Nietzsche quote here but then remembered an earlier thread where it can be found, too: