When creating a new calendar, you can choose to have iPhoto include event information from iCal. You can also add iCal data to an existing calendar by clicking the Settings button and checking the appropriate Import iCal Calendars box.
Don’t use iCal to manage your life? You can download thousands of calendars from iCalShare (www.icalshare.com). Because I like my calendars to include the phases of the moon, I subscribed to a lunar calendar at iCalShare. When I’m creating a new calendar, I simply check the moon-phases calendar and iPhoto does the rest.