Putting real products under there would not be a very smart move. It only takes a minute or two to quickly hide something else instead before the reporters arrive.
Why lie? Why cover empty boxes? They have enough goodwill and fanboyism without having to manufacture an aura. They curate it, no doubt. Which leads me to the hand-picked reporters. Calculated, definitely.
The reporters a risk? Nah. If Gruber yanked a cloth, he'd gain notoriety, be escorted out and others would glimpse something in a relatively similar form-factor or not at all (say, the iPad a year ago).
It would suck for Apple. But can you imagine the expectation over an iPad being revealed from a PR event went wrong? The expectation would be even higher.
There is little risk. They can manage. It seems like more work for Apple to lie at this juncture.
Even more awesome if he then yanked off a mask a la Mission Impossible to reveal that he was really Steve Ballmer all along.
None of the reporters had a camera, and most cloths probably weren't covering anything serious.