The Linksys 54G stuff uses the Broadcom chipset, and Broadcom won't release any programming information about their chips.

Your best bet is try one of the various projects attempting to provide a wrapper around the original binary driver.
Check out
http://www.linuxant.com/driverloader/
and
http://ndiswrapper.sourceforge.net/
I think they both require a fair bit of effort though.

Other than that you could write a nasty letter to Broadcom letting them know what you think about their policy of excluding Linux.

THO