Clicking 'Check guest and resource availability' brings up a 'find a time' dialog which indicates which times are free / usy in the calendars of each attendee. It's essentially the same functionality that you get when you choose the 'Scheduling' tab in Outlook.
I've only seen this in the Google Apps calendar so far, and it's a welcome addition.
UPDATE: Tony Ruscoe points out that he saw this feature months ago when Google Apps Premier was released. So it's more of a case of rolling a feature out to more users rather than a new feature. Add a comment if you're seeing this feature and if it's new for you.