If your LAN and WLAN networks are on the same subnet, you can use a trunk(4) interface for seamless switching back and forth. For example:

Wired interface /etc/hostname.re0 (just up, no IP):

up

Wireless interface /etc/hostname.ral0 (same here, only link settings):

nwid myssid
pwakey secret
up

Trunk interface /etc/hostname.trunk0 (including IPv4 and IPv6 autoconfiguration):

trunkproto failover
trunkport re0
trunkport ral0
dhcp
rtsol autoconfprivacy

Here is re0 a prefered and ral0 a fallback interface (order is important). I.e. the trunk switches seamless to the wireless interface if the wired interface gets disconnected and back to it if the link is available again.