I happen to own one spare Linksys WP54G wireless card. When inserting it into the Laptop, FreeBSD complains:

cardbus0: Expecting link target, got 0x3
cardbus0: Resource not specified in CIS: id=10, size=2000
cardbus0: Resource not specified in CIS: id=14, size=20000
cardbus0: <network> at device 0.0 (no driver attached)

But behold! There is a solution: It basically consists of compiling your own NDIS driver. This procedure is also known as "Project Evil". It's very straightforward, so I will outline the steps necessary for the installation of an WPC54G, EU version:

• Get LSTINDS.INF, LSTINDS.cat, FwRad16.bin and tnet1130x.sys from the Linksys CD-ROM. For your convenience, I uploaded these files (a note to Linksys here: please don't sue me for I am helping people using your great wireless cards under more operating systems. This will surely increase your shareholder value - sooner or later.), so you can grab them.
• Extract all of these files execept FwRad16.bin into a directory of your choice. FwRad16.bin has to be placed in /compat/ndis/.
• Start ndisgen. This text-interface will guide you through the installation if you choose "Convert Driver". If prompted for the .INF-file, specify the path to LSTINDS.INF. If prompted for the .sys-file, specify the path to tnet1130x.sys. Additional files are not needed.
• You should now have the file tnet1130x_sys.ko. This file should be placed in /boot/kernel/.

Assuming you want to load the card drivers at startup, place the following in /boot/loader.conf:

ndis_load="YES"
if_ndis_load="YES"
tnet_1130x_sys_load="YES"

I haven't tested the lines in /boot/loader.conf because I use a shell script for loading the device drivers and connecting to my favourite access points.