Last year when I preordered my 3GS, the same thing happened to me and many other people. I was told it had shipped and the delivery date was a day early. That stuck for a while while it was in transit. This generated a lot of exciting discussion here on Macrumors.
I started to think it was really going to happen. We were leaving for a trip on launch day, so I'd only get it if it did arrive early, otherwise it would sit in a box for five days until I got back.
Well, to make a long story short, it sat in a box until I got back from my trip. Late in transit, just a day before delivery, the shipment went "on hold" and waited patiently for the launch date.
It will likely happen this way again. Just warning everyone.