Combining Calendars in iCal

This is really quite simple, but not simple in the usual Apple fashion.

1. Create a new calendar group.
2. Drag the calendars you want to combine into the group.
3. Select the group, and then export it via File->Export
4. Import the the file you just created into a new calendar.
5. Delete the calendar group after being happy that everything is present (which it should be).

That's all there is to it.