For folks who really want this functionality, there are links to detailed instructions on using a Linksys 802.11g card for Mac Portables (also instructions for desktops)
http://www.macosxhints.com/
article title: Use Airport Extreme in Older Macs
For anybody wanting to get 802.11g on the cheap in their older Macs Nick Sayer is your new savior. He has a site of tips and tricks for OS X and recently mentioned that he has gotten a Linksys 802.11g card working in previous portables. Yes, you too can get the WPC54G working in a portable or possibly get the WMP54G in PCI Macs.