No one outside of Apple, and likely very few inside of Apple, if anyone at all at this early stage, knows what the 2012 iPhone will be called. For the sake of convenience and familiarity, we've been using iPhone 5 (or whatever Apple ends up calling it).
But Apple can call it anything they want. iPhone 4G. iPhone 4GS. iPhone 4GS Mark II Epic, LTE X. (Okay, that's very unlikely, but it's their phone, their name.)
There's no logical reason they couldn't call it iPhone 5 still. The model number will be 5,1 after all, and for most mainstream customers the logical number after 4 -- even after 4S -- is 5. They don't know, and don't care, what generation the device is, nor should they.
Apple doesn't really care either. They've used 3 twice and 4 twice now. They could easily use 5 twice next. They still call iOS 5 on Apple TV "software 4", after all.
Marketing has nothing to do with reality and everything to do with perception and presentation.
There will almost certainly be a new iPhone next year, but until they officially announce it and it's name, we'll be using iPhone 5.
Update: "The new iPhone."