As you've probably realized already, getting WPA to work on the WL-138g isn't as simple as you'd think...
For some unclear reason, ASUS does not provide any new driver update for this card.
And Marvell, who produced the Libertas chip on this board and wrote the drivers, clearly state that they refuse to support end users.
But the silliest here is that a working driver DOES exist !
So here it is: http://nicolas.melay.free.fr/libertas.cab
This is driver version 126.96.36.199, dated from 06/08/2005.
Yes, this is the same version number as the x64 driver you can find at support.asus.com, but it's a real and working 32-bit driver.
I had heard from it, I couldn't find it anywhere on the web, and it finally poped in while I was testing Vista and visiting the Windows Update website (using another network adapter of course). I just picked up the file from the Windows Update cache directory (C:\WINDOWS\SoftwareDistribution\Download).
Note that there are both NDIS 5 and NDIS 5.1 driver files, so it might even work on 98/Me/2000 with the appropriate wireless config tool.
I have no idea how it will work with ndiswrapper under Linux either. Any feedback here would be welcome.
None of the older Marvell drivers ever got me anywhere with WPA.
This one worked like a charm.
I hope it will for you too.