Letty wrote: Somewhere I read that Ponce de Leon's name was really pronounced Pontha because one of the Castilian kings had a speech impediment. Can't substantiate that, however.
I think that's a legend.
Linguistic studies show that during the late XV and the XVI Century -that is, the time of the unification of Spain-, there were diverse ways in pronouncing the c, the s and the z. This is quite documented, since there was no official Spanish spelling at the time, and words were written as they were pronounced. Letters from different conquistadores would systematically have different spellings. The main "th" influence, it seems, comes from Judeo-Sefardí, the version of Spanish spoken by the Jewish community in the XV Century.
Regional evolution of the language standarized one single spelling and two broad sets of pronunciation.
Today, standard Castillian (spoken in Castille, Navarra, Galicia, Catalonia and Valencia) uses "correct ceceo": pronounces "ci" and "ce" as "the", "thi" (with the "thorn" th).
Andalusia, Extremadura, the Canary Islands and the whole of Latin America use "correct seseo": pronounce "ce", "ci" as "se", "si".
Rural zones of Andalusia and Extremadura, and Spanish gypsies, use "incorrect ceceo": pronounce any s before a vowel as "th".