iOS 5 iMessage Won't Activate

I had updated my iPad 2 to the iOS 5.0. Everything works fine except that the iMessage app won't activate. When I try to sign in to iMessage using my Apple ID, it displays the error "Could not sign in. Please check your network connection and try again." The iPad has connected to WiFi network with no problem.Tap Settings > General > Restrictions, then disable restrictions. iCloud need to be on as