An Interjet Airbus A320-200, registration XA-RBA performing flight 4O-435 from Tijuana to Leon (Mexico), was enroute at FL370 about 200nm northwest of Leon when the crew received an ECAM message indicating excessive cabin pressure/altitude. The aircraft descended to FL110 and continued to Leon for a safe landing about 35 minutes later. Mexico's Accident Investigation Board rated the occurence a serious incident and opened an investigation. The AIB reported the crew performed a precautionary descent to FL110 following a "CAB PESS ALT EXCESIVE" ECAM message.