While the iPod and the iMac cannot compete with budget alternatives on price, they are nevertheless carefully balanced hardware/price-wise. The ergonomics and styling are the obvious selling points and what persuades the buyer to pay a LITTLE extra; the hardware used is neither "unique" nor best-of-class.
Thus, any screen beyond 800x600 is also a breaking-point stretch of any imagination: it would be too expensive. Touch-screen: as already stated, Apple has no hardware-design capabilities of that kind in-house, and the color variety is also too expensive for the price range Apple wants to aim for.
Apple is dedicated to industry standards these days; the few standards they have initiated recently (FireWire, AirPort, DVI), have been developed in careful cooperation with major industry players. Which port would be standard for mounting/inserting a camera optics? The one used by Ericsson in their new GPRS model? To my knowledge, it's their own standard, not adopted by any other producer. The Handspring bus might be an option, but again it's not a widely adopted standard.
3-D interface? UI research has shown that any 3-D interface elements are a hindrance until the screen format is larger than 21" and the resolution at least 1860x1600. It is NOT useful for a PDA or near-PDA, neither with respect to screen area nor ergonomically. Why was Palm so successfull? A very basic UI - the lesson carefully learnt from Newton research. There is also the matter of hardware: it would take a better dedicated GPU - adding heat and battery drain. The interface elements which are part of QT is a much more viable option here.
The interesting speculations thus don't work at the present stage; neither G4 or G5-derived embedded chips have the general capabilities needed for a PDA, and they are still too hot, power-consuming and expensive. In three years' time, Apple may have the know-how and access to the inexpensive parts needed to build what you suggest; not now.
But I agree with Matrix Agent - the hardware combo I envision could easily become an extension to all Apple's i-software: iPhoto, iMovie, iTunes - AND mail (downloading this morning's mail and reading it on the train or metro; hooking the iScreen up to the Mac and answering it there). As for AirPort connectivity, I believe that is for a later stage as is touchscreen. Again, economics - and power consumption. But it would be lovely...