According to some sources, the Burj Dubai is now the world tallest building:

Burj Dubai is now officially the tallest, man-made structure in the world. The tower scaled 510 metres to beat the Taipei Towers that stands at 507.3 metres. Official sources engaged in the construction of the world's tallest tower told The Gulf Today that they have reached the goal in terms of the building's height. Although the height of the skyscraper has been kept under wraps, it is believed Burj Dubai will end up somewhere between 705 and 950 metres. When completed, it will have between 154 to 180 floors. Construction for what is set to be Dubai's landmark structure began on Feb 1, 2005 and is set to be completed in June 2009. Being constructed by Samsung Corporation, the tower is designed by Chicago-based Skidmore, Owings and Merril. The tower is the centrepiece of the Dh73 billion Downtown Burj Dubai Project of Emaar.