Hi:

Yes, the card I used works under Classic Mac, even with Apple's stock Airport software. It even does 128 bit encryption under OS 9.x.x

How did I find it? I looked and researched very hard. The following instructions tells you everything you need to know, even the model number of the wifi card - the Orinoco OEM wifi PC Card. Couple it with an iBook G4 antennae, and you are off to the races.

http://www.thisoldmac.ca/wordpress/2009/05/03/newton-messagepad-20002100-and-powerbook-2400c-internal-wifi-upgrade/