Since 2004, the world's tallest building has been the Taipei 101 skyscraper in Taipei City, Taiwan, at 1,670 feet (508m), which took the lead from the 1,483 foot (452m) Petronas Twin Towers in Kuala Lumpur, completed in 1998. Neither of them is the world's tallest structure; that title goes to the 1,815-foot (553m) Canadian National Tower in Toronto, which has been the world's tallest free-standing tower¹ since 1975. The title of world's tallest tower is scheduled to be stripped from the CN Tower by 2010, the expected completion date of Japan's planned New Tokyo Tower, planned to be 1,970 feet (about 600m) tall when completed in 2010.
The New Tokyo Tower, though, won't get to set any record but "tallest freestanding tower" for even a day. Two years before its scheduled completion, the Burj Dubai is planned to become both the world's tallest skyscraper AND the world's tallest structure of any kind, free-standing or otherwise. The exact planned height of the building, being constructed by Samsung, has not been publicly disclosed, but the promoter and developer of the tower says it will be "more than 700 meters (2,296 feet) and more than 160 stories", overtopping the New Tokyo Tower by 100 meters and Taipei 101 by 200 meters.
Think about that for a moment. A building 700 meters tall. That's almost half a mile. I don't know about you, but I think the race for one kilometer is on.
Here's another thing to think about: When the World Trade Center was completed in 1977, it stood just over a quarter mile (not counting the rooftop antenna spire). It took 11 years to build (construction began in 1966). The Burj Dubai will be almost twice as tall, yet is projected to be completed after only four years (construction began in 2004).
[1] There are higher non-free-standing masts; the KTHI-TV Tower in North Dakota is 629 meters tall, but it is cable-guyed.