Welcome to LQ!
What chipset does your wmp54g v4.1 use?
It might be a bit of a pain, but if you try a KNOPPIX LiveCD
you should be able to run "/sbin/lspci" in a terminal from it
and see what chipset is being used.
Prelimary investigation says it uses the RaLink RT61 chipset.
If so, that is in the 18.104.22.168 kernel that Slackware -current
uses, and the firmware was added on March 11.
You can use it with Slackware-12.0. However, you might need to
compile a new kernel, and install the firmware. If you wait for
Slackware-12.1, it will more than likely work when you install.