The origins of the Calderon surname are quite varied, and difficult to verify.  Legend holds that the first man to bear the name was called Fortun Ortiz Calderon.  It is said that he was a still-born baby, and his parents, thinking him dead, placed him in a nearby caldron.  The child "awoke" and alerted his parents with his cries.  From then on, the last name Calderon (spanish for caldron) was forever attached to him.  This is said to have happened in the town of Nograro, Spain around the start of the 13th century. 

Several sources continue to tell the tale of Fortun into old age.  He is said to have been the illegitimate child of Fortun Sanchez de Salcedo, the sixth lord of Ayala.  He went on to become a powerful knight and "ricohombre" of Castile in the service of King Alfonso X.  His lands included the towns of Nograro, Oteo, Quincoces, Gorunde & Villamaderni.  He had four children, Francisco (prior of San Juan convent), Sancho (comendador of Santiago, died fighting the moors and given credit for the family motto), Ana (sometimes called Elvira) and Maria.  Fortun died after spending the last years of his life leading his clan in a war against the Angulo clan.  His remains are said to be buried in the Santa Maria de Herrera monastery (between the towns of Aro and Miranda).  Most of his story is told by his son-in-law Lope Garcia de Salazar (married Ana / Elvira), known as the first historian of Biscaya (present day Basque territory).  Sadly, it is not believed that any of Fortun's sons had children or where able to carry on the family name.

update (1/25/08) - Up until know, everything I read did not list Fortun having any grandchildren.  But I recently read the 'Diccionario Historico, Genealogico y Heraldico de las Familias Ilustres de la Monarquia Espanola", which states that Sancho Ortiz Calderon married Maria de Zamudio, and that they had four children: Hernan Sanchez Calderon, Teresa Ortiz Calderon, Alvaro Sanchez Calderon and Rui Sanchez de Calderon.  So it seems that old Fortun's story is not yet complete.  I'll try to piece more together and keep updating.

Meanwhile, other historians dismiss this story as myth without fact.  They consider a verifiable forebearer of the Calderon's to be Pedro Ruiz Calderon, born around 1213.  He is first mentioned in the documents of the Abbey of Santa Maria Aguilar de Campoo, in the north part of the current province of Palencia near the border of Cantabria.  He also had an extensive family, and I am still researching him.

Of course, it could all be as simple as the name being associated with metalworkers who worked on caldrons.  During the middle ages, it was quite common to give a man a name related to his profession.  I will keep looking and post when I find new developments.

Family Motto: I will die for the faith / Por la fe morire.  Legend holds that Sancho Ortiz Calderon uttered these words as he refused to renounce his faith when captured by the invading moors.