dhclient fails to assign an IP address to wlan0. The WAP is using wpa-psk
for security and wpa_supplicant successfully associates to the AP. Wait
time on the machine in question exceeded 12 hours without the interface
being assigned an IP address. Using "/etc/rc.d/netif restart" failed to
resolve the issue.

tcpdump on the wlan0 interface showed it to be operational, outputting
broadcast packets between the AP and other hosts on the network.

A lease file for the wlan0 interface was apparently successfully created,
and showed the expected information for the network, including host IP
address, nameserver address, etc.

Forum postings suggested using synchronous dhcp and some reported success.
However on this machine that technique resulted in the system hanging on
boot waiting for dhclient. Research into dhclient configuration yielded
no suggestions for resolution.

Fix: 

A successful workaround is to configure the interface for WPA and static IP thus:
ifconfig_wlan0="WPA inet x.x.x.x/24"
and adding the appropriate information to resolv.conf.

A multihomed system may need to have the default route explicitly defined in rc.conf:
defaultrouter="x.x.x.x"
How-To-Repeat: Configure the wireless interface to use WPA and DHCP and reboot.