Before I begin, I should tell you that this only happens in iOS 5.1. Before the most recent update, this had never happened and it still does not happen on any othe
It sounds like you're removing some sort of private preference, which is probably a new bug, either with NSUserDefaults (you shouldnt be able to remove it) or UIWebView (it should cope with a missing entry).
Have you tried the method from the other answer (setting a blank dictionary?). Does that give the same results?
How about if you get the dictionary representation of NSUserDefaults, get all the keys, and iterate through those, removing the objects? (let me know if you need a code sample for that).