Of course she is not.
No, she's not. I know another Goldsmith who is not Jewish. either way, this is the proof she is not:
Kate Middleton's ancestry is documented in detail here -
http://www.wargs.com/other/middleton.html
Her last traceable "Goldsmith" ancestor was a carpenter named John Goldsmith who was born in Kent around 1783. At the time, there were a skant few Jews living in England, and pretty much all of them were Sephardic Jews who didn't have names like "Goldsmith". On the other hand, "Goldsmith" can easily be a non-Jewish English last name (as can many other names starting with "Gold", including "Gold" itself), and John's son married in a Baptist church.
My guess is, based on her family tree, is that Prince William probably has more Jewish ancestry somewhere down the line than Kate Middleton does.
Furthermore, she is actually very, very faintly descended from Royalty. Her great-grandmother x14 was Anne Gascoigne Fairfax. Anne's great-grandprents x5 were Edward III and Phillipa of Hainault. Other notables on her father's side include some 17th c. clergymen. There is also a more tenuous connection to another so-many-great grandparent called Henry Davenport who was descended from Queen Elizabeth I and whose wife was the great-great grandaughter of Anne Boleyn.
On her mother's side, the Goldsmith connection is listed in Burke's Peerage (
http://www.thepeerage.com/p20299.htm). Whilst the fact that the marriage took place in Holy Trinity Church and not a synagogue leads me to think they aren't jewish. If you click on the paternal links and follow the ancestors back, they were all married in churches or register offices and lived in modest houses