Why internet shopping didn't change the world.
No matter how convenient online shopping may be, it's just that - a convenience. Because you still have a human on one end of the transaction acting as a bottleneck. Until you see shopping agents that will not only find you the best prices, but actually figure out what you need and go connect to other machines to actually buy it for you and just have it arrive before you knew you needed it, online shopping will remain a convenience (albeit a tremendously convenient one).
Of course, there's a tremendous amount of infrastructure that needs to be solidly in place before this happens, but I'd find it hard to believe that there are any barriers that couldn't be overcome with current technology.
That, and you've got to trust the machines not to randomly decide you want a few hundred pounds of silly putty.