There is no WPA support in the ACX100 driver (yet).
There is wpa_supplicant for BSD/Linux systems, which adds software-side support for it (as intended by the WiFi alliance).
The wpa_supplicant doesn't add WPA support to the ACX driver, but it is possible use ndiswrapper in order to have WPA support.
Explanation on why the plain (i.e. non-softmac) ACX driver not support WPA.
Roadmap for the addition of WPA support
The SoftMac version of the ACX driver (acxsm) was going to be the version with WPA support. After the mac80211 stack became the official kernel wireless stack, development switched to this new one, and the softmac development was abandoned. This section is therefore only here for historical purposes.
The code was even added to the wireless-dev kernel tree (not part of the mainline kernel), and it can still be accessed from John Linville's 2.6.x legacy wireless networking tree (tiacx branch). This code should only be used for reference purposes.
The ACX driver used to live under the tiacx directory in the wireless kernel tree. Also see 
TODO: add some information on Devicescape, add links to linuxwireless
Mac80211 was formerly called d80211 (and also referred to as dscape or Devicescape), and some time ago Jeff Williams announced the first version of a d80211 port of the acx driver on the acx100-devel mailing list.
The mac80211 stack was added in the the 2.6.22 mainline kernel.