But only once before. Apple has been very aggressive with their SoC plans, actually. Looking at this move, I don't think the AppleTV alone has the volume to justify a die-shrunk A5. So, you have two scenarios:
1) Shrunk A5 finds it way into 4S to cut costs and improve battery life. Next iPhone gets new chip.
2) Shrunk A5 finds it way into 4S to cut costs and improve battery life. Next iPhone gets same A5.
2) is very possible, but given Apple's aggressive track record, I'd imagine they would want to keep pushing the envelope and do something new. iOS 6 unveil will be a huge hint as to what to expect. If it does a lot of new, compute heavy things, a new CPU seems more likely to me. Of course, a lot of those special features may not be unveiled until the new iPhone is, as they would remain exclusive to it.
No, it would require a firmware (BIOS) hack, if the core wasn't hard disabled or non-functional.