With the rumours that Apple may launch a new iPhone with a bigger (physical screen size) display, isn't there a chance that the 'nano' will just become based around the existing size?
I mean, touchscreens don't work so good when you go too small (for text entry accuracy), so we'll probably get a newer, slimmer, iPhone with 4+ inch screen and the current design becomes the nano, or a 'classic' model.
I'm only guessing based on ergonomics, but given that Apple doesn't release stuff that is badly designed or hard to use (one thing, I'd have to admit that some cheap OEM manufacturer wouldn't hesitate to do), it seems like common sense.
The problem Apple has is that no matter how many crappy Android devices are released, there are an increasing number of excellent ones to suit different tastes and needs. We have a range of screen sizes, rugged and durable models, a gaming model, ones with keyboards and so on. Literally something for everyone.
Apple has one (maybe two) models a year and that's about it. I also think that the biggest threat to Apple will be when people simply start to get bored of Apple and fancy a change. Sure, Android will have that same problem in the future, but it happens to everyone at some point.