5.4 / 6.0 wi0 and dhcp with closed network

[JK]

I cannot get this to work at startup time on 5.4

On 6.0 I put  ifconfig_wi0="ssid mynet DHCP" into /etc/rc.conf and 
that works.  I end up on the closed 'mynet' network via a DHCP 
address.

On 5.4 neither that line nor other combinations in /etc/rc.conf of

ifconfig_wi0="DHCP"
ifconfig_wi0="ssid mynet"
dhclient wi0

work.  I _can_ get up on 5.4 by booting and then manually doing

ifconfig wi0 ssid mynet
dhclient wi0

After dhclient receives the IP etc from the server the machine is up 
fine.

How in the world do I do this in /etc/rc.conf ( or other files ) on 
5.4 so it works automatically at startup time?