While glancing through a 2009 Taurus catalog I noted that they make a 9mm compact with a forged titanium slide. They state that they are the only gun manufacturer that knows how to forged titanium. As far as I can determine they are correct- other parts on other guns I know of are made from investment casting or bar stock.
A slide is a fairly complex part to forge and machine so they must be on to something. Titanium is a great material for a lot of firearms uses - I'd love to see it used for a 1911 frame and some S&W revolver frames.