iPhone 3G: "Twice as fast, for half the price"
iPhone 3GS: "The fastest, most powerful iPhone yet"
Apple certainly will market a new iPhone based upon speed alone. The A4 is a system-on-chip that is basically made up of the same components as the 3GS (except for additional RAM). The A5 has a more modern GPU and CPU. Now if only they cover up the antenna or something...
I said they won't sell the phone simply on a speed increase. An iPhone isn't a computer, they need new features to entice users to upgrade. Even if NFC is the only feature added, it's still an additional, marketable feature. The iPhone 3G had 3G and GPS. The 3GS had an improved camera, compass, voice control, etc.
There are probably not going to be any major changes in the iPhone 5, but it still has features that aren't being spoiled by the iPad 2 introduction.