The Wi-Fi Alliance says 25 devices certified with WPS: The new security standard uses a simplified method to distribute a WPA/WPA2 encryption key to a device that wants to join the network. The standard was nailed down last year, and it was expected that WPS would hit the market earlier than this. Apple's AirPort Extreme Base Station with 802.11n that shipped in February includes WPS, but not under that name; it requires an 802.11n adapter and Mac OS X to negotiate via WPS.
With WPS, a user presses a physical button on a router or uses a gateway configuration tool or Web browser to click a software button to initiate the "next device gets a key" connection method. A short PIN can be used instead. In Apple's version, you use its AirPort Utility program to add a client, choosing either the next computer to connect or the PIN option. If the latter option is chosen, after you choose the network from the client, the client produces a PIN which you enter in the utility software. It sounds more elaborate than it is.
No word on XP or Vista support baked in, unfortunately.