Move all events from one iCal calendar to another
Select one of the calendars you want to merge
choose File->Export...->Export
Make sure the filename matches the calender you've chosen above. The calendar will be exported to a .ics file
Choose File->Import...->Import and find the file you've just saved.
ical will ask you to which calendar it should add the events. Just select the another one of the calendars you want to merge.
repeat until all calendars are merged.