How to sync multiple calendars (Exchange/Google) on a Mac?
Spanning Sync ($25/year) should do this. However, when I tried to use it I kept getting a crash (version 3.2/443). 

BusySync ($39.99) should also do this [edit: I just got a note from BusySync support saying that this does not work for Exchange]. 


  You may now add and edit events, in iCal or Google, on any of the calendars you published or subscribed to, and the events will sync in both directions between iCal and Google.


However, when I set up my Exchange calendar to publish to Google, and modified the calendar inside the Google Calendar interface, the new modifications get clobbered the next time BusySync syncs. It does at least successfully publish the events on Google Calendar, though.