After the NHK started to broadcast TV back in 1953, a large tower became a necessity to broadcast the analog waves over the Kanto region which lead to the construction of the 332.5 meter tall Tokyo Tower in 1958.
In 2011 however Japan is going to ditch analog TV for terrestrial digital broadcasting. The current height of Tokyo Tower is not adequate to cover the region with digital broadcasting which is why the Tokyo Sky Tree is being constructed - a tower 634 meters tall thats due to open to the public in 2012.
The Tokyo Sky Tree is obviously problematic for Tokyo Tower as all the broadcasting companies are planning to jump ship and pay for the antennas on the Sky Tree.
Because of this, the folks who run Tokyo Tower are planning on increasing the height of Tokyo Tower by 100 meters which involves increasing the actual structure height by 20 meters. Am presuming that the rest will be made up of some long extension at the top.