TechCrunch has posted an interesting photo and write up of the Apple Tablet that never was -- the Pen Mac, unreleased in favor of the PDA-style Newton back in 1990.
The Pen Mac was a fully functional Mac computer (it even played the Mac startup chime) with a pen based touch screen. The screen itself was identical to the Mac Portable, but with the addition of pen touch. And of course the case was a lot smaller than the Mac Portable. The Pen Mac was supposedly not much more than one inch thick. Users could plug in a keyboard and mouse or easier input.
So, it happened before will it happen again? From Newton to iPod touch. From Pen Mac to... iTablet? We'll have to wait for 2010 to find out.