WHILE AWAITING TAKEOFF CLEARANCE, THE CAPT STATED THAT HE OBSERVED A FLASH OF LIGHT & SMELLED SMOKE. HE INITIATED THE PAX EVACUATION CHECKLIST & ALL PAX EVACUATED THE ACFT. EXAM OF THE ACFT BY COMPANY MAINT PERSONNEL REVEALED THAT THE NO. 1 GENERATOR CONTROL UNIT (GCU) LOCATED BEHIND THE FIRST OFFICER'S SEAT EXPERIENCED INTERNAL FAILURE. THE GCU WAS REMOVED FOR FURTHER EXAMINATION. EXAM OF THE UNIT REVEALED THAT A DIODE LOCATED INSIDE A TRANSFORMER WAS SHORTED, WHICH OVERHEATED THE TRANSFORMER & PRODUCED THE SMOKE. ACCORDING TO BOEING, THERE HAVE BEEN 12 SIMILAR OCCURRENCES.