Yes, the tablet concept have been around for ages, but all of them (I'm supposing you're talking about the early Microsoft concepts and other mid-90's ones) was created BEFORE THE ADVENT OF WI-FI networks and mobile internet (3G and so on). So of course such a kind of device was pointless and there were no market place for them.
The iPad changed everything. And yes, I followed /. discussions at the time of iPad release and the consensus was that the tablet device was still pointless as the other attempts before, so the iPad would fail miserably. Of course, ignoring the new form-factor, the touch-screen technology, the app ecosystem, and the mobile internet availability. In few weeks, 10 million people bought a pointless device to discover a way to use it in a near future.
I thing that /.ers have a kind of child-trauma to admit the succeed Apple market strategy, and the boldness they have to break established paradigms (this no-USB cries == no-floppy in the 97' iMac, no-optical devices on MacBook Air, and so on).
The iPad was never, in any moment the *first* tablet, but was the first with huge success and Apple was the first company to bring it to the masses. Of course being first in an entire new market gives Apple the advantage to be always ahead of competitors: when all these Android tablets come out, the iPad2 is just around the corner.
The problem of being an Apple hater is that the competition doesn't help very much.
Yes, The iPod predictions here is the Slashdot's 9/11.