You're right. I was wrong.
The Italian specs page is misleading. (plain wrong, actually)
This is the original US page:
So:
iPhones (3GS/4): tap to focus AND tap to exposure (you know, when you tap on dark everything gets lighter, when you tap on a white wall everything gets darker, etc)
iPod Touch 4G: only tap to exposure, probably no room for a proper autofocus mechanism there.
So the iPod Touch 4G cannot tap to focus apparently.