OOB there is no way to get an overview where reusable content was used. When content editors change such a content item, they often want to see if the pages where it was inserted are still correct. However, this data is stored in SharePoint 2010 (should be also present in 2007):

using (SPSite site = new SPSite("http://intranet.local"))
{
  using (SPWeb web = site.OpenWeb())
  {
     var listId = new Guid(web.AllProperties["_ReusableContentListId"].ToString());
     SPList reusableContents = web.Lists[listId];
     foreach (SPListItem content in reusableContents.Items)
     {        
        Console.WriteLine("Links for content item: " + content.Title);
        foreach (SPLink link in content.BackwardLinks)
        {
           Console.WriteLine(link.Url);
        }
     }
     Console.ReadKey();
   }}

I am going to implement this as a ribbon button, which will show a modal dialog with all BackWardLinks of that item.