If you have multiple calendars set up in Apple’s iCal application and you receive an invite from a friend, family member or client, you can easily decide which calendar you want the invite to be placed in. Simply hold down the Accept button, rather than just clicking on it, to get a pop-up menu which allows you to choose which iCal calendar it should be placed in.
This doesn’t seem to work with Exchange calendars – holding accept or opening the invite only lists my built-in calendar and not the corporate exchange calendar.