Actually sublime1, if you include the antenna Sears is now the tallest. The World Trade buildings used to be.
There are 4 ways to measure it.
1. Height to the structural top. (Includes spires but not antennas)
2. Height to the top occupied floor.
3. Height to the top of the roof.
4. Height to the top of the antenna.
The winner of the first three is Taipei 101 in Taiwan.
Sears Tower has the fourth.
The Petronas Towers no longer holds any, but prior to this year it held the first.
All the info here.