It's pretty much considered a legend. The term for it in Spanish is ceceo.

http://ccat.sas.upenn.edu/~haroldfs/540/handouts/ideology/silvrstn/MSLVRSTN.htm
http://www.phatnav.com/wiki/wiki.phtml?title=Lisp_(speech)

I'd never heard the Thames story, and it seems unlikely:

http://www.bartleby.com/61/11/T0141100.html