Try out Calgoo (http://www.calgoo.com) which is free.
Publish each iCal calendar to the calgoo hub servers. Then, set up a GCal account you’ll use to aggregate them all. Subscribe to each calendar you published on Calgoo within the GCal account, then…
stop This isn’t going to work… bah… maybe take a look and see if Calgoo would help in any way but the feed from a Google calendar only includes GCal created entries, rather than subscribed calendars too, which is unfortunate because otherwise you could have used it for aggregation. There may still be a way to do this but I have to go to work!