Unfortunately the graphical tool doesn't quite cut it, so everything here requires editing text files and shell commands. Hopefully not too complex though.
There are two files that you need to alter (everything else is ready to go):
/etc/wlan/wlancfg-<yourSSID> (copy this from wlancfg-DEFAULT)
Edit wlan.conf to change the following two lines at the bottom of the file:
(yourSSID must match the file name of the second file as above)
In the wlancfg-<yourSSID> file, set the network-specific settings, and leave the rest as-is. Here are the important lines from mine:
dot11WEPDefaultKey0=XX:XX:XX:XX:XX (I'm not telling you this...)
(you'll see that I use WEP on an ad-hoc network)
Then start the connection from the shell:
ifconfig wlan0 <fixedIPaddr> up
(use whatever <fixedIPaddr> you like; I haven't tried DHCP but I've seen that others have it working and it should operate the same as any other interface)
Stop the connection with:
I saved some scripts and linked them to menu items and buttons to make life easier with the up and down, and also some routing. Hopefully I haven't missed anything; you should be able to go by the comments in the files if I have.