I know I'm probably in the minority, and call me old-school but I would love to have something very basic like an ipod nano with a clickwheel and phone functionality. I could easily live without the internet features, touch-screen, wide-screen, movies, photos, etc.
I don't know price-wise how much this would make a difference but you could drop:
wifi & bluetooth connectivity
high-res widescreen
touch-screen interface
sensor to detect widescreen or vertical orientation
sensor to detect if it's close to your face to turn off touch-screen
ambient light sensor
horsepower to run OSX light, and battery/power management to run it.
While not having a keyboard would hurt text messaging ability - I don't use this much anyway.
I know this takes away all that makes the iphone so compelling and high-end, but it would be a basic apple solution to the problem of not wanting to carry both a phone and an ipod around.
Alternatively you could make it a serious full-sized ipod with an 80gb hard-drive, instead of a nano- flash drive size.
Likelihood of apple doing this? probably close to nil, but I bet there would be a market. On the one hand with apple's current cool-factor, and with Skype VOIP solutions, some people just need a basic cell to be contacted when away from home...
call me a minimalist - at least until the iPhone prices come down