marcelorocks:
I was having this problem with Arduino IDE 1.0.5. I noticed that going back to 1.0.3 works. Looks like something is bad between 1.0.5 and wifi shield.
THANK YOU! You just saved me.
Indeed, I confirm that the WiFi shield that refused to work corectly since I bought it last thursday (and I paid a hundred of Euros for it!) now works like a charm since I've downgraded the IDE from version 1.0.5 to 1.0.3.
Thank you @marcelrocks, you definitely rock!!! 