There is titanium used on the commercial airplanes that I have worked on (Boeing, Douglas, Lockheed, Airbus). There are two basic types used on these airplanes; commercially pure titanium, which is surprising not to strong and is used where structural strength is not a big requirement; and titanium alloys which are used where structural strength in a tight spot is a requirement. The most commonly used titanium alloy is one with 6% aluminum and 4% vanadium as the alloying components.
This alloy basically has the weight of aluminum with the strength of steels. Titanium is expensive and difficult to work and form, but its modulus of elasticity is almost the same as aluminum while steel is 4 times higher; so in critical applications where strength and space along side aluminum parts is a requirement, titanium is better than steel for good fatigue life. The modulus of elasticity is an indication of how much the material will flex under load, if one material will flex more than another, then the stiffer material is more likely to crack over time. This is a common problem where steel sheet is used to reinforce aluminum skin around door corners, for instance. Titanium is much better both in the initial design and as a repair material, but the cost and difficulty in forming discourages many from its use.
Also, there is a lot of titanium used in the compressor sections of turbine engines; here the high strength with low weight is the reason for its use.