See also my original article on this subject.
I've just read an interesting scholarly article by Michael Heiser on monotheism and polytheism in ancient Israel. He notes that it is widely accepted that Israelite religion was originally polytheistic:
[S]cholars... frequently assert that no explicit denial of the existence of other gods occurs until the time of Deutero-Isaiah and thereafter (6th century B.C.E.) in a presumed campaign by zealous scribes to expunge such references from the sacred text. Even the Shema and the first commandment do not consign the other gods to fantasy, since the demand is made that no other gods should be worshipped.
He believes that this widespread view does not go far enough:
It... fails to handle the evidence of late canonical and non-canonical texts that “retain” a council of gods in Israelite and Jewish theology.... There are explicit references to gods and a divine council in Second Temple period Jewish literature.
Essentially, Heiser takes the view that passages in the Old Testament which have frequently been interpreted as teaching monotheism (in Deuteronomy, for example) actually affirm only the uniqueness and incomparability of Yahweh. This is an interesting and rather persuasive idea, though Heiser perhaps goes too far in rejecting entirely the categories used by modern scholars - monotheism, polytheism, monolatry, henotheism - to analyse ancient Near-Eastern religion.