NetworkWorld has a great pull of a Jony Ive deposition which references looks at early iPad prototypes which were much thicker, made of plastic and buttonless on the face:
During the course of the deposition, Ive at one point is shown photos of a three dimensional mockup of a tablet that Apple produced as part of the discovery process. It’s referred to as the 035 mockup or prototype.
Ive is then asked if he’d seen the mockup before, and if so, when.
He responds that he has seen the 035 mockup before and adds:
My recollection of first seeing it is very hazy, but it was, I’m guessing, sometime between 2002 and 2004, some but it was I remember seeing this and perhaps models similar to this when we were first exploring tablet designs that ultimately became the iPad.
So the takeaway is that Apple was tinkering with tablet prototypes as early as 2002.
Network World was then able to find the actual pictures Ive was referring to in unsealed court documents (above).
You’ll recall that Steve Jobs actually said that Apple worked on the tablet long before moving to the iPhone (which appears to be from 2002 on – right after the release of the iPod). However, Apple stopped working on the iPad to focus on the iPhone which was, at the time, more important. Eventually, “when Apple got its wind back in its sails”, Apple “took the iPad off the shelf and used what they’d learned in making the iPhone to make the iPad.”