Google has added support for syncing up to 25 calendars with Windows Phone 7.5 devices. Simply visit m.google.com/sync from your Windows Phone to start what is a very easy process. Select your device, the calendars you’d like to sync and then save. Within a half hour, all of your appointments and events from Google Calendar will show up on your Windows Phone. As an added bonus, you’ll also be able search Gmail messages, even those which haven’t been downloaded to your phone.
Source: Windows Phone Blog