Super Mario 3D Land will use Streetpass for the exchange of items and other things and assist blocks will provide special items inside levels. The information was derived from the back of dummy packages from the Japanese versions of the game.
@Guillaume Died on the next one? Perish the thought. Seriously though, Mario 3 had pretty good balance. The P Wings were very rare (on the first playthrough at least) and the cloud had the chance of being all for naught. What a great game!
But the items in this game are not like Mario Galaxy; those were contextual. In this game, you can keep items, like in the 2D games. Maybe they'll make an exception for this one, but if they don't, it'll be the dumbest thing they've ever done.
Well what about the people that want to use them to get past particularly tough areas, just because the option doesn't suit you doesn't mean someone else won't want it. I don't think they would have outed a powerup purely to put in the P-wing.