Marko Savic argues that the iPhone does have sub-brands, just in a slightly different manner:
3GS, 4, 4S are used colloquially as much as nano, shuffle and touch. As long as there is only one new device on offer per year this naming convention works. The meaning of each iPhone sub-brand changes over time from premium, to mid-market, to low-end, moving back up to premium in the pre-paid market. It’s disruptive to traditional product branding.
Apple is taking an operations-oriented approach to product life-cycles.