I had a fun task of finding an alternative solution to an Aeronet 350 PCMCIA card that is still available and is still being manufactured that would run on a Windows CE 4.2 operating system. It sounds like such a simple task... but no... it has taken me about 6 months to find any sort of wifi solution that would do this. However we did have a bit of a power constraint as well, so that may have hindered things a bit too!
Anyway, what I have managed to find is a PCMCIA to CF converter and a CF Wifi card 802.11g that has wifi drivers for it for Windows CE 4.2. It looks like a promising solution, it is low power, it does more than the old 802.11b card and it is made by Socket.
Datasheet - http://www.socketcom.com/pdf/default.asp?Document=Datasheet&Product=WiFi-P500
User Guide - http://www.socketcom.com/pdf/default.asp?Document=User%20Guide&Product=WiFi-P500
Quick Start Guide - http://www.socketcom.com/pdf/default.asp?Document=Quick%20Start&Product=WiFi-P500