It is not true, the Titanium case design lasted two years, the Aluminium case design lasted five and a half years and the current case design is just shy of being two and a half years old.
Proof: www.everymac.com or any other site chronicling Apple's models, which could be found via some search engine, like Lycos or AltaVista or Bing or Yahoo or MetaCrwaler or ... .
The 15" and 17" MacBook Pro have only been around since 2006. It's current "unibody" design was released in 2008. 2009 saw the new 13" MacBook Pro, which was made from the 13" unibody aluminium MacBook.