The firmware for the WiFi shield has changed in Arduino IDE 1.0.5. You are recommended to install this update per these instructions
With the Arduino WiFi Shield, this library allows an Arduino board to connect to the internet. It can serve as either a server accepting incoming connections or a client making outgoing ones. The library supports WEP and WPA2 Personal encryption, but not WPA2 Enterprise. Also note, if the SSID is not broadcast, the shield cannot connect.
Arduino communicates with the WiFi shield using the SPI bus. This is on digital pins 11, 12, and 13 on the Uno and pins 50, 51, and 52 on the Mega. On both boards, pin 10 is used as SS. On the Mega, the hardware SS pin, 53, is not used but it must be kept as an output or the SPI interface won't work. Digital pin 7 is used as a handshake pin between the Wifi shield and the Arduino, and should not be used.
The WiFi library is very similar to the Ethernet library, and many of the function calls are the same.
The WiFi class initializes the ethernet library and network settings.
The IPAddress class provides information about the network configuration.
The Server class creates servers which can send data to and receive data from connected clients (programs running on other computers or devices).
The client class creates clients that can connect to servers and send and receive data.
The UDP class enables UDP message to be sent and received.
Corrections, suggestions, and new documentation should be posted to the Forum.
The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.