I'm a few months late on finding this out. That is what I get for not checking the forums or the product vendors website on a regular basis. Either way the WMP54G wireless card now includes WPA encryption security. There are few more things that have been added as well. To get the driver update follow the link below.
WMP54G – Wireless-G PCI Adapter v4 Linksys Driver Downloads
Driver Date : 3/15/2005
This update is working smoothly for me so far. I just installed this new driver earlier this morning and gave it a spin for an hour or so. The machine it's in use on is an older machine running Microsoft XP Professional.
I previously covered this card and my frustration with it's lack of security options in this article.