The story...

Titanium discovered in metal and dust samples at the WTC may (in combination with the presence of other chemicals) indicate the use of thermate.

Our take...

Professor Steven Jones quotes the following article as an explanation of how thermite reactions may be detected:

“When thermite reaction compounds are used to ignite a fire, they produce a characteristic burn pattern and leave behind evidence. These compounds are rather unique in their chemical composition, containing common elements such as copper, iron, calcium, silicon and aluminum, but also contain more unusual elements, such as vanadium, titanium, tin, fluorine and manganese. While some of these elements are consumed in the fire, many are also left behind in the residue.”
http://www.materials-engr.com/ns96.html

In analysing dust and metal samples later, he identifies high levels of Titanium, and asks “Where does it come from?”

Good question. How is it answered in other studies? This one has a couple of suggestions:

...particles rich in iron, aluminum, titanium, and other metals that might be used in building construction...
Possibilities include metals that might be found as pigments in paints (such as titanium, molybdenum, lead, and iron)...
http://pubs.usgs.gov/of/2001/ofr-01-0429/chem1/

Another suggests concrete could be a factor. We’re not so sure about that -- wouldn’t that be more common in reinforced concrete than the type used at the WTC? -- but for what it’s worth:

Metals present in very fine particles in relatively high concentrations were iron, titanium (some associated with powdered concrete), vanadium and nickel (often associated with fuel-oil combustion), copper and zinc...
http://www-dateline.ucdavis.edu/021502/DL_wtc.html

A third proposes paint as the answer:

The very high levels of titanium (> 0.1%) were due to their presence in paint, especially white paint...
http://www.ehponline.org/members/2002/110p703-714lioy/lioy-full.html

There’s a mention of Titanium use at the WTC site here:

The base of the [new Freedom] tower, which was originally to be clad in polished steel and titanium like 7 WTC...
http://skyscraperpage.com/cities/?buildingID=7788

And Encarta lists many uses of Titanium, uncluding (of particular interest here) its use in aircraft, “paints, lacquers, plastics, paper, textiles and rubber”:

Because of its strength and light weight, titanium is used in metallic alloys and as a substitute for aluminum. Alloyed with aluminum and vanadium, titanium is used in aircraft for fire walls, outer skin, landing-gear components, hydraulic tubing, and engine supports. The compressor blades, disks, and housings of jet engines are also made of titanium. A commercial jet transport uses between 318 and 1134 kg (700 and 2500 lb) of the metal. Titanium is also widely used in missiles and space capsules; the Mercury, Gemini, and Apollo capsules were made largely of titanium. Other common titanium alloys include ferrocarbon titanium, made by reduction of ilmenite with coke in an electric furnace; cuprotitanium, formed by reduction of rutile to which copper has been added; and manganotitanium, produced by reduction of rutile to which manganese or oxides of manganese have been added.

The relative inertness of titanium makes it available as a replacement for bone and cartilage in surgery and as a pipe and tank lining in the processing of foods. It is used in heat exchangers in desalinization plants because of its ability to withstand saltwater corrosion. In metallurgy, titanium alloys are employed as deoxidizers and denitrogenizers to remove oxygen and nitrogen from molten metals. Titanium dioxide, known as titanium white, is a brilliant white pigment used in paints, lacquers, plastics, paper, textiles, and rubber.
http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761569280

Where did the Titanium come from, then? We don’t know, however it seems there’s plenty of choice without having to invoke thermate as a possibility.