WiFi can be thought of as "wireless Ethernet" and is a standard way of connecting PDA's and other computers to local area networks. All cards listed below are based on the 802.11b wireless standard unless otherwise noted. The Compact Flash format has been around for years and comes in two physical sizes. Type I cards are smaller, while Type II are slightly larger. Quotes are from the respective companies webpage and are not a value judgment of davespda.com.
 

Belkin F5D6060
"The Belkin 802.11b Wireless PDA Network Card easily implements networking into PDAs by installing into any available CompactFlash Type? II expansion slot and sets up easily in Windows CE 3.0 or higher." - www.belkin.com

Hawking HCF30W
"With this 11M Wireless LAN CF+ Card, you can access your network wirelessly for high speed Internet browsing and E-mail anytime, anywhere in the office."
www.hawkingtech.com

Linksys WCF11
"Experience the same speed and ease of wireless networking with your CompactFlash Type II ready personal digital assistant as you do with your laptop or desktop computer with the Instant Wireless Wireless Network CF Card."
www.linksys.com

Linksys WCF12
"Experience the speed and convenience of wireless networking on your PDA. The Wireless CompactFlash Card installs directly into your Pocket PC using a CompactFlash Type I or Type II slot."
www.linksys.com

Sandisk Connect
"The Connect CompactFlash Wi-Fi Card (802.11b card without memory) allows Laptop and PDA users freedom to access the Internet, check email, send and receive from any available access points (hotels, airports, coffee shops, conference centers)."
www.sandisk.com

Sandisk Connect Plus 128
"SanDisk?s 128MB + Wi-Fi CompactFlash card combines flash memory and wireless communications into a single card giving PDA and notebook users the freedom to quickly transmit and receive data, images and music from thousands of hotspots worldwide."

Socket WL6004
"Now you can access the Internet, check e-mail and transfer files and information wirelessly using Socket's Low Power Wireless LAN Card. Connect easily to enterprise or public 802.11b Wi-Fi WLAN infrastructures or directly with others using the Ad Hoc Mode."

Sony PEGA-WL110
"Wireless LAN card for CLIO handheld."
www.sonystyle.com

Symbol LA 4137
"The Wireless Networker from Symbol Technologies brings the power of high-speed Wi-Fi wireless connectivity to today's popular personal data appliances."

Feel free to email me about your own PDA, questions, comments, articles, rumors, and reviews.  I can be reached at dave@davespda.com.