CNet.com presents the current view of free wireless networks and security: the article notes that Windows XP supports 802.1x, a method of using certificates to authenticate users across a network. Microsoft offers an extremely cogent explanation of its 802.11 and 802.1x featureset. 802.1x requires three things: client support (to initiate a session, provide identity information, and handle encryption), access point support (to negotiate with the client and a RADIUS server for user authentication), and a RADIUS back-end. RADIUS is an industry-standard method of authentication for logins that was developed initially to support large banks of modems and other remote-access to networks.
An excellent explanation with graphcis from Network World Fusion on 802.1x
Updated 802.11b security article: I've added in details about 802.1x among other things to the Wireless run-down.