Which makes sense, because iPhone 6 just sounds silly. Just make it like all of their other hardware, where they are just "The New {blank}" when released, and then referred to by their timeframe/year for those who care (Late 2011 iMac, Early 2010 MBP, etc). It will just be "The New iPhone" by name, and people will just refer to it as the 2012 iPhone when differentiating.
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But, it isn't the 5th iPhone, so why would they do that? In recent years, the iPad 2 and iPhone 4 were the only products that actually had a sequence number in their name. Going by the iPad's new, simpler name, it would stand to reason that the iPhone will follow suit.