Slackware has a wpa_supplicant package in the extras directory. It isn't installed by default, but it is easy to add once you've got a system running.
why they dont bundle this baets me as teh code for teh driver (madwifi) is GPLed
The problem is that it isn't entirely GPL'ed. There are bits and pieces (HAL in particular) that are rather confusing from a GPL standpoint. Have a read here
for some basic info, but the upshot is that the licensing has been confusing enough that most distros don't want to touch it.