Taipei 101 is currently the world's tallest building (November 2007).
The following image shows a comparison between Taipei 101, the Sears Tower, the Petronas Towers, and the Empire State Building:
The Sears Tower is taller from ground to spire, but Taipei 101 is taller from ground to roof. The Burj Dubai is taller than Taipei 101, but will not be completed and occupied until 2009.
The following image shows a comparison of the Burj Dubai with Taipei 101 and other tall buildings of the world:
Images from Wikipedia, under GNU Free Documentation license.
Tallest Building in the World
I thought China was building the world's tallest building in Shanghai. Is the one in Dubai going to be bigger than that?