If users with Outlook 2K10 don't share their calendars properly, users of older versions of Outlook (2K3, 2K7) can't view the shared calendars. Attempts to view the shared calendars return pop-up messages claiming insufficient permissions.
The user of Outlook 2010 should follow this procedure.
Go to the File option on the top, then the Info ribbon on the left, then Account Settings button, and select Delegate Access from the resulting drop-down.
Click Add… and select the target user from the Address Book that opens. Click OK.
You should now see that user's name in the Delegates window. Highlight the name and click Permissions.
The desired permissions are “Reviewer” for the Calendar, and “None” for the other categories. Click OK to accept the permissions, and OK to dismiss the Delegates window.
Now the user of the older version of Outlook should be able to view the shared calendar.