I agree with Gruber that if Apple is releasing a "tablet" next month, it'll be more towards the iPhone side of things: a specific OS and UI designed for doing all of the little computing things you do with a portable computer. You don't necessarily sit down to edit video while curled up in bed or sitting on the train, just like you don't edit video (though you can) with your iPhone.
But you do read, browse the web, check and write email, watch video, maybe play some games. The phrase Gruber eventually attaches to all of this is "personal portable general computing," and I think he's dead on. While the iPhone is a suitable replacement for most of this, there's an upper limit on it as a smartphone. But the tablet, with a specialized UI, OS, and even App Store-style apps, can fit that category perfectly.
Very interesting writeup. Gruber says early in the post that the iPhone was a complete secret before it released, and that's not quite true -- we all knew there was going to be an Apple phone eventually, even if we didn't know specifics (and yes, some of the guesses were silly). And I think likewise, we all have a good idea already of what an Apple tablet will be like, even if the specifics of the device will end up surprising us after all.