iOS 5 insists on "Update Apple ID", but won't accept my e-mail address
I'm trying to update my iPhone 4 to iOS 5.
After the update, iOS 5 begins to ask a series of questions (would I like navigation services turned on, etc.)
Eventually it asks for my Apple ID and password. I enter those, and it accepts them.
Then it asks for my birthday. I enter that.
Then it show a screen that says "Update Apple ID: For your security, your Apple ID login must be an email address."
But my Apple ID / iTunes ID isn't an email address. If I enter the e-mail address associated with my iTunes account, it says "Try Another Apple ID. This Apple IS is already being used."
Of course it's already being used .. it's mine!
What's going on here?