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The final stages of bringing the product to market may require greatly expanding the circle of those who are "in the know", making preserving secrecy more difficult. Steve wants to control the revelation, not some leak to do it.

One key reason for the usual 'right now' availability is to avoid cannibalizing sales of existing products. They were happy to pre-announce the iPhone because there were no existing Apple phones. Arguably the tablet can also be pre-announced for the same reason (although one could also make an argument in the other direction viz the MacBooks)

I doubt it; content and software would not be ready in time.

The iPhone SDK had a six-month public seed time for devs to create their apps for the opening of the iTunes app store, but there's been no public SDK for the tablet yet. It'll be announced now, out in June.

The iPhone wasn't released with an SDK or App store though. While I think it would be unlikely for Apple to launch a tablet / new mobile product that doesn't have an app store, it wouldn't be the first time.