I would like to be able to default sharing the user's calender to everyone using a group policy. Is this possible?
A brief google hasn't come up with anything.
(Outlook 2010 and the AD is Server 2008)
You can't set this in group policy, you would set it on the Exchange server. Here is an example: http://blog.powershell.no/2010/09/20/managing-calendar-permissions-in-exchange-server-2010/
Are you sure you actually want to share everyone's calendar? That's sort of unusual, considering the meeting assistant shows busy time already when scheduling.