Checkout this thread: [SOLVED] Thinkpad T400 Wireless
Helped me get wireless setup. I would strongly suggest installing WICD - works fine in xfce, makes life easier.
As root #slackpkg install wicd
Try this in order to keep track of you have done.
Disconnect your DSL cable
Make sure your laptop wireless button is switched to "ON"
Reset your configuration files to their original state by running "netconfig" and enter hostname then choose loop back. Thast it.
As root make rc.wireless executable
check /etc/rc.d/rc.wireless.conf has "Pick Any Access Point Section is Uncommented. Don't put anything in config file.
vi /etc/rc.d/rc.inet1.conf uncomment: use dhcp = yes, essid, host, wlan key, wpa supplicant
If you use a password write in the wlan key = prefixed by s:
Then just click on your wicd icon and click on your Wireless Network. See if you connect.
If you don't get a wireless connection. You can use WICD feedback to post here and figure out what is to do next. This worked for me, after days of going crazy