Is MAC whitelisting anything but security theater? Isn't it trivial to determine a valid client MAC then spoof it?

What makes you say that? It does not seem trivial at all to guess a valid MAC.

	▲	tirant an hour ago \| parent \| next [-]
		The MAC addresses of all the Wi-Fi clients are broadcasted in plain radio format all over the 2.4GHz. It is trivial.
	▲	ProllyInfamous 2 hours ago \| parent \| prev \| next [-]
		It's not just a guess. Any decent sniffer (e.g. airsnort) can immediately identify all associations between all WiFi/Bluetooth devices. DD-WRT (router firmware/OS) has this WiFi-associations detector built-in ("local WiFi map"). There is no need to attempt any sort of hack — associations are publicly-broadcast information. Then, just pick any authorized MAC and duplicate as your own.
	▲	0x457 an hour ago \| parent \| prev [-]
		It's in managmenet frames that you can sniff.