Titanium is a great material for use on bikes, similar properties to
steel but lighter and it oxidises nicely. It is usable both in parts
(XT hubs have Titanium axles, for example) and in frames.
It also has 2 strong drawbacks:
1) Cost.
2) Difficult to work and weld.
The second is really a product of the first. Many Al alloys, for
example, need great care when welding. The problem with Titanium is
that it's high cost leads to limited use and hence techniques and
experience are limited.
But Titanium is not expensive because it is scarse, it's quite abundent
in the form of Titanium Oxide. The costs are in the refining which are
both difficult and produce toxic waste.
But hope is on the way, this article presents a new method of refining:
http://www.techreview.com/printer_friendly_article.aspx?id=16963
The article seems focussed on aerospace applications, but if it works
out then it could be great news for cyclists as well.