In the past, I setup my Gmail and Google Calendar sync with iOS, Windows Phone, and Symbian devices using the Exchange ActiveSync protocol. Google posted a Winter Cleaning post on the Official Google Blog stating that syncing via EAS is ending starting 30 January 2013.
Google is also discontinuing Google Calendar Sync and Google Sync for Nokia S60 so any remaining Symbian users may want to start thinking about a smartphone upgrade for the holidays. Existing users may be fine for a while so if you have it setup now, don't switch devices or shutdown the service on your device.
Here is the statement regarding the EAS protocol:
Google Sync was designed to allow access to Google Mail, Calendar and Contacts via the Microsoft Exchange ActiveSync® protocol. With the recent launch of CardDAV, Google now offers similar access via IMAP, CalDAV and CardDAV, making it possible to build a seamless sync experience using open protocols. Starting January 30, 2013, consumers won't be able to set up new devices using Google Sync; however, existing Google Sync connections will continue to function. Google Sync will continue to be fully supported for Google Apps for Business, Government and Education. Users of those products are unaffected by this announcement.
As Kevin Tofel recently wrote, you can currently setup Gmail and Google Calendar on Windows Phone 8 using the Google option and m.google.com/sync to view multiple calendars. I am sure Gmail will still work fine after the EAS shutdown and your own calendar likely will as well, but I am not sure if the multiple calendar support (very handy for families) will remain supported with these upcoming Google changes. Kind of makes today's Engadget editorial a bit more convincing, doesn't it?
Google supports CardDAV to offer syncing via IMAP, CalDAV, and CardDAV, which I believe means that iOS users should not really be affected, unless they setup their account using the Microsoft Exchange protocol. Shoot, given the excellent Google apps for iOS you are likely better off using those rather than syncing through some protocol anyway. However, Google is currently not really supportive of Windows Phone. I used to use the Google Calendar Sync tool to sync my Google personal calendar with my work Outlook calendar, but that support is going away soon too. It looks like many of these services will continue to work for existing users, but no new devices can by configured and the services will not be actively supported so will likely eventually die out.