It's returning 404 right now: http://www.apple.com/library/test/success.html

When iOS can't connect to this page, it puts up a log on screen that only goes away after the page can be reached or the user presses cancel. Pressing cancel disconnects the user from the network.

As it stands, I and a couple of friends can't use WiFi on some of our iOS devices.

(The sporadic nature of this makes me wonder if these devices are supposed to be checking a different location and aren't for some reason.)

More info on success.html: http://erratasec.blogspot.com/2010/09/apples-secret-wispr-request.html