Two weeks ago, Apple and AT&T outlets opened their doors to massive lines of fans eager to buy the new iPhone 3G. The stores were stocked and ready to sell, but the network — shouldered with the burden of activating so many new phones — was quickly overwhelmed, and the process slowed to a crawl. Is this sort of pile-on some sort of preview of what hides around the corner with the surge of video downloads on the Internet at large? Is the Web’s infrastructure inching toward collapse?