Xcode 4.5 corrupting XIBs?
I had this same issue and fixed it by unchecking the 'Use Autolayout' checkbox on the Document Inspector pane in Utilities.
Uncheck the "use Autolayout" highlighted in the below image. Xcode 4.5 enables this property by default for the new nib files you add into your project. Unchecking the autolayout check box solved the problem
I just had this problem and tracked it down to a UIButton that had the Title field set to Attributed in the Attributes Inspector. Changing this to Plain fixed the problem. If I remember correctly I added this button in the last version of Xcode and had trouble with it then also.