Breaking news from Google: Google Calendar now supports Apple iCal.
It is not really a solution for synchronising with Exchange, though a very useful extension to Google calendar. With a couple of tweaks it will be possible to synch Google calendar to Exchange too. The only feature missing that prevents Google calendar to be a proxy between Exchange and iCal is free/busy service. I am afraid that the only solution for fellow Mac users is to wait until Snow Leopard is released, and hope that the new incarnation of iCal would support Exchange natively.
Follow the link to the official Google blog above to find the instructions on how to use Google calendar with iCal.