I doubt next years iteration will be called the "iPhone 5". Something along the lines of how the 3G went to the 3GS. It'll be a minor tech bump and something will be done about the antenna to shut people up but nothing too drastic. My 2 cents -- which are worthless -- thanks for reading.
I can't really see them naming it the iPhone 4S or 4.5. With the release of the iPhone 4 they have put a definite numeric stamp on the phone itself. It seems to me like they will have to continue using the numeric stamp from now on, or at least if they want to be consistent.