I have a .mac Apple ID. That’s what all my DRMd iPhone apps and my music is tied to. It’s no longer a valid email address, I got it back when that’s how Apple IDs worked.
Your Apple ID and password are two key parts of your Apple Account. You can use your Apple ID and password in iTunes to sign in to the iTunes Store, buy content, and authorize items you've purchased. If you already have an Apple Account, you can use your existing Apple ID and password to sign in and buy and authorize items you've purchased from the iTunes Store. Your Apple ID must be a valid email address, for example "email@example.com." Your password must be at least six letters or numbers, and is case sensitive.
Changes you make to your Apple Account while you're in iTunes are also recognized by other applications where you use the same Apple Account (for example, the online Apple Store, MobileMe, or iPhoto). You may be asked to verify your information the next time you use your Apple Account to purchase something in another application…
What’s new here is they’re documenting what happens when you change you Apple ID. In theory you can now revise your Apple ID “while you’re in iTunes”. But wait, the My Info page is viewed from a web browser, not iTunes. They’re not really saying that you can change your Apple ID and it will all work nicely with iTunes and the App Store.
Be very afraid.
Credential transitions are the sort of thing people screw up routinely, and Apple has a record of botching it.