The recently pushed-out beta of iOS 5.1, aside from trying to fix the issues and bring new Siri commands to the table, also contains important information about the upcoming iPhone. Those who took a peek at the code saw mentions of a certain iPhone 5,1.
We’ve gotten somehow used to the iPhone 5 name even though it’s only been just rumors, until now. iPhone 5,1 means that there will be a phone which Apple internally calls the “iPhone 5” and the “,1” would mean an initial release/variant of the phone. A follow up to the iPhone 4S (say iPhone 4GS for the sake of argument) would bear a codename of “iPhone 4,x”.
This is by no means a guarantee that the next iPhone will indeed be called the iPhone 5. Apple might use a completely new name, it’s just that we’d find it convenient (and logical).