Though the iPhone 4 and iPhone 4S look the same, it seems like the AT&T iPhone 4 and the iPhone 4S are just different enough—the silent switch and volume up/down buttons are shifted ever so slightly—that your old cases won't fit.
The iPhone 4S button layout is a lot like the Verizon iPhone 4 (though we can't confirm if they're exactly the same) and those cases needed to accomodate for the button shift. There were cases that worked for both models, yes, but if you had an AT&T iPhone 4 and a case that didn't have an open gap for the iPhone's left side buttons, you'll proobaaably need to look for a new one. And here you thought Apple didn't change anything! [Android-Sale via UberGizmo]