Use Exchange Calendar with Google Apps
If your organization is switching to Google Apps from Microsoft Exchange® and some of your users want to continue using their Exchange calendar, they can do so with Google Apps Calendar Interop for Microsoft® Exchange. With Calendar Interop, some users can use Google Calendar to manage their meetings and events, while others can use Microsoft Exchange. Calendar Interop syncs the free/busy status of users in both calendar systems so everyone can see each others' schedules.