Switching wireless networks: WPA <-> unencrypted

[Lars Stokholm]

I usually use two WPA encrypted wireless networks and I have
wpa_supplicant.conf set up for that. In rc.conf I have
ifconfig_ath0="WPA DHCP". It's working fine, but once in a while I
have to use other, unencrypted networks and that presents a problem.
How do I connect to these with the least amount of trouble?

Right now it involves commenting out the line in rc.conf, restarting
netif, manually configuring ifconfig and running dhclient.

Surely there must be a better way to do this...? Preferably it should
automatically fall back to using unencrypted networks, if no
configured encrypted networks are available... or something like that.