On all my trips to Greece I can't recall seeing it at any of the places we went where plates were broken except once when a waiter dropped one in Corinth - but apparently its expected by tourists searching for that "real" Greek experience. There are many explanations for that particularly Greek custom as an expression of "kefi" - the outpouring of joy, excitement and passion. Some say that it derives from ancient traditions and there are other schools of thought that suggest it has more modern origins.
Many thanks to Cathy for passing this on and to my darling Yannis, Fotis and Marius in Athens - Μου λείπεις τόσο πολύ!
26 settembre/September - Santi Cosma e Damiano