I would say the only substantial change from one iPod to the next was the amount of storage and the quality of the screen. You'd never need to talk about which generation you had, because they only do one thing: play music. Meanwhile, successive iPhones have a few features that differentiate how they actually behave:

    - processor (screen smoothness, webpage performance, what games you can play)
    - camera (quality of images)
    - radio antenna type
    - OS versions supported (iPhone features often vary by OS version, unlike the features of the traditional iPod)

In conclusion, iPods only have one real feature, and it doesn't change, so you don't need to talk about version numbers. iPhones have more features that do change, so version numbers are important.