The symptoms arrived with this command:
# /etc/rc.d/wpa_supplicant start ath0
ioctl[SIOCG80211, op 98, len 32]: Invalid argument
ELOOP: remaining socket: sock=5 eloop_data=0x800e0b1c0 user_data=0x800e070f0 handler=0x421840
/etc/rc.d/wpa_supplicant: WARNING: failed to start wpa_supplicant
The solution, add these lines to /etc/rc.conf:
You no longer directly access the wireless device. In my case, I have the following chipset (as discovered via pciconf -lv):
ath0@pci0:3:0:0: class=0x020000 card=0x058a1014 chip=0x1014168c rev=0x01 hdr=0x00
vendor = 'Atheros Communications Inc.'
device = 'Atheros AR5212 802.11abg wireless (AR5212)'
class = network
subclass = ethernet
I found this post and Colin Percival
confirmed that the solution had worked for him.