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 t

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)