We have been using switch-interface since build 668, so yes, it does work, and has to either be done at the CLi (as above, after removing ALL DHCP servers, Firewall policies, etc.) or built in a full script.
 
 Here is ours, offering one wireless for internal users (on the same subnet as the LAN), and another for guests.
 
 config system switch-interface
     edit " NewInterfaceName" 
             set member " internal"  " wlan"              
     next
 end
 config system interface
     edit " wlan" 
         set vdom " root" 
         set wifi-ssid " InternalUsers" 
         set wifi-security WPA
         set wifi-passphrase ENC  *see caveats*
     next
     edit " internal" 
         set vdom " root" 
     next
     edit " wan2" 
         set vdom " root" 
         set status down
         set type physical
     next
     edit " wan1" 
         set vdom " root" 
         set ip x.x.x.x y.y.y.y
         set allowaccess ping https ssh snmp
         set type physical
     next
     edit " modem" 
     next
     edit " ssl.root" 
         set vdom " root" 
         set type tunnel
     next
     edit " NewInterfaceName" 
         set vdom " root" 
         set ip 10.0.0.1 255.255.255.0
         set allowaccess ping https ssh
         set type switch
     next
     edit " GuestWireless" 
         set vdom " root" 
         set ip 192.168.1.1 255.255.255.0
         set type wireless
         set wifi-ssid " ExternalUsers" 
         set wifi-security WPA
         set wifi-passphrase ENC *see caveats*
     next
 end
 
 *CAVEATS*
 
 Because it is now in a switch, the wlan is no longer a stand-alone interface.  Therefore, you lose the ability to manipulate ANY portion of it in the GUI.  If you want to restart it, you have to CLi it.  If you want to change the SSID, or the passphrase, you have to CLi it.  Broadcast/silent? CLi.
 
 Also, because of the above, it is not possible to change the passphrase in plain text, so you will have to find other means to determine the encrypted hash.  I would suggest you create a third guest wireless, set the passphrase you want in the GUI, then go to the CLi, highlight the hash, and paste it into your wlan.  Then remove/disable the 3rd guest wireless.
 
 Hope this helps!