I have an app built under iOS5. When I compiled the app with a few updates I discovered that no UITextFields in the app would acccept text input. The keyboard would appear, but typing would have no effect on the UITextField. My MainWindow was built using IB instead of via code.
Searching SO I discovered one possible problem was [self.window makeKeyAndVisible] missing from the application delegate, however, that did not solve the issue for me as I already had that in my application delegate class.
Answering my own question to spread the knowledge...
I found this post in the apple forums that provided the answer:
In interface builder select the MainWindow.xib file. Then select the viewController (in my case a TabBarController) and view the Attributes Inspector. There should be a field under View Controller which says "Title." I placed the name of my MainWindow xib ("MainWindow_iPad")