There is now a convenient way to query free-busy information from Google Calendar. The new Google Calendar free-busy feed allows you to query blocks of busy time for one or multiple users, or for all the users subscribed to a Google Group in a single request.
This new Google Data feed is accessible through the URL pattern: https://www.google.com/calendar/feeds/default/freebusy/busy-times/userID -- (where "userID" is the email of the calendar you'd like to request the free-busy information of.) The response will be an entry containing temporal blocks indicating the periods when the user is marked as busy in their Google Calendar.
You can also query free-busy information for multiple users in a single query using a batch request. The URL to use for such a batch request is: https://www.google.com/calendar/feeds/default/freebusy/busy-times/batch
Google Apps for Business domain users are also able to query free-busy information for all the members of a given Google Group in a single request. This feature is available through the following URL pattern: https://www.google.com/calendar/feeds/default/freebusy/group/groupID/busy-times -- (Where "groupID" is the email of the Google Group.)
The response will be a feed containing the free-busy entries of each member of the group. For more details and code samples about the free-busy feed, visit this Developer's Guide, and to learn more about querying free-busy for groups please visit this Guide.