Why Titanium for Bicycles?

Because titanium is extremely strong, very light and because it will not rust, it is used in the manufacture of high end bicycles.

Titanium density : 4.52g/cm3.

Titanium Grade 5 (Ti-6Al4V)

Fasteners are popular in the industry of racing motorcycle and automobiles for its high tensile strength and lightness, it is mainly used in racing car, motorcycle and bike applications.

For more information:
http://wiki.answers.com/Q/Why_is_titanium_used_in_bike_frames#ixzz1Jc7ahDDX
http://www.wrenchscience.com/WSLogic/Frames.aspx?store=titanium&stylecode=R