I'm not sure what they will call the next iPhone... but using a numeric identifier makes sense for several reasons.
Originally Posted by Garz
Apple currently sells 3 different models of iPhone... they have the "flagship" model that they sell for top dollar - currently that's the iPhone 4S. Then they have the phone that the carriers sell for $99 - currently the iPhone 4. Finally they have the "free with contract" iPhone - currently the 3GS. When the new iPhone comes out, the 3GS will go away and everything bumps down the ladder one step.
Using numeric identifiers is not nearly as important to the other products that Apple sells, because they only sell one version of each... this is true for the iPod (iPod Touch, Nano, Shuffle), the iPad (iPad and the soon to be announced Mini) and computers (15" MBP, 13" MBP, Retina MBP; 27" iMac, 21.5" iMac; Mac Mini). Sure, they sell different versions of each device, but each model is refreshed (or retired) with the entire product line... so there's no real reason to differentiate the current year models from the previous versions. Apple uses a relative identifier to mark the refresh timeframe (early 2012, mid-2010, etc). However, when the product line is refreshed, the older models are retired and no longer sold (after an initial "fire sale" effort).
So I firmly believe that the next iPhone will definitely carry some type of identifier to denote it's place in the hierarchy of iPhones...