Does anyone have any insight into why the shadow, introduced in Supplement I: Greyhawk, and whose original description specifically notes that they "are not 'Undead' per se" came to be numbered among the undead in the AD&D Monster Manual? Holmes muddies the waters a bit by transforming Greyhawk's clearer language into "Shadows are not turned by clerics," but he nevertheless doesn't call them "undead creatures" as the Monster Manual does. Interestingly, Moldvay is quite explicit about the fact that shadow are not undead, thus bringing it more in line with OD&D's conception of the monster.
In my Dwimmermount campaign, shadows are not undead but malevolent manifestations of Chaos found in areas tainted by it. Still, I find it more than a little odd that Gygax reversed himself so radically on this question. It's a minor point, admittedly, but it bugs me nonetheless and I wish I'd thought to ask him the question back when I was corresponding with him.