I have both an iPod Touch and an iPhone - did a bit of research and the main difference is that the iPhone contains parts which implement the mobile phone functionality, otherwise known as the baseband processor. The iPod doesn't have those parts nor does the iPad.
Both iPod and iPhone are very compact implementations. Have you looked at the tear-downs on iFixIt.com ? I don't think it would be possible to alter an iPod without creating a bulky, Frankenstein-ian device. Of course, for some people that would be a plus, bit like the steam punk movement.
If you ever go any further with your idea, then by all means let us HN readers know about it.
