In my opinion, this just proves that Apple sometimes has no coherent naming policy at all. While the iPhone 4 was the first iPhone to be named after its generation, the 4S already broke that rule by referring to its technical aspects instead of its generation (a faster iPhone 4, hence 4S). If you compare this to the iPad, while the second iPad followed the iPhone naming policy, the third iPad was simply called the new iPad instead. So even for the iPad the naming policy has changed.
Because of this, I think that Apple tends to come up with a name that fits depending on the circumstances. In the media, there have been so many claims about this iPhone 5 even before the iPhone 4S was announced. Looking back at the overall disappointment that was expressed, the public may still expect a true iPhone 5 after all, even though it would make no sense generation-wise. Considering this, I suppose Apple could just go along with the name that everyone is expecting anyway.
It could also be that Apple is just playing with us. Apple has on several occasions placed subtle hints on its invitations. It could be that the 5 is just a hint to the next iPhone, since everyone is expecting the iPhone 5, thus symbolically pointing to the iPhone. That does not mean that it could not have a different name.