Yes. None of them were widely commercially successful though. Products that come to mind include the Crunchpad (which I'm not sure actually made it to market), the Always Innovating Touchbook and the Pepper Pad. Of these, the Crunchpad was most similar to the iPad.
So no, Apple didn't invent a new product area with the iPad. They were just the first to find commercial success in that product area.
So, I'd argue it's still a huge improvement over an existing product area, with the added bonus that it created a market. Products existed, but no significant market for them was available.
The iPhone's improvements over Blackberries and the like of the time were as impressive as the iPad's improvements over Windows tablets, for example, but one doesn't feel the iPhone a new product area.
See e.g. http://www.linuxfordevices.com/c/a/Linux-For-Devices-Article...
It wasn't as sleek as the iPad, of course - battery, LCD and touch technology has come a long way, but it worked quite well.
And we were not first with a custom tablet UI (the article above compares it to the Ericsson Screen Phone for example: http://www.linuxfordevices.com/c/a/Linux-For-Devices-Article... )
Yes tablets had existed in the past but did any of them have scaled down, touch optimised UIs ? All I recall is full blown Windows which was probably the biggest reason they didn't succeed.