Apple did not release any comment on the issue when it cropped up on online forums, so trust users of iDevices to come up with a workaround. In theory, iOS 4 would ping a page on Apple’s servers (http://www.apple.com/library/test/success.html), and if successful, the Wi-Fi was deemed to have connected, and everything is hunky dory. Needless to say, if the ping failed, the system would think you were on a network which needed a login and proceed with credentials authentication. It seems that Apple deleted that page from the server by accident, resulting in failed pings and subsequently, the inability to hook up via Wi-Fi. The page was restored since, and the issue seems to have been fixed. Were you one of those affected earlier today?