Apple has made some absolutely incredible, and successful, products: the ipod, the iphone, and of course their desktop computers.
Some products haven't been so hot, like the Newton. Its been years since that product was first launched and subsequently canned, but I got the feeling that its failure left Apple with such a bad taste that they would not try it again.
Rumors suggest the contrary - "iSlate" is supposedly a tablet in the works, about to be launched, by the Apple.
Technology has changed, evolved, improved since Newton, even the iphone would easily outperform it.
I see some critical success factors for an Apple tablet to succeed: