There are a couple of known issues with Excel 2007 that result in the #REF! error value being returned from links. You'll want to read both of the following Microsoft KnowledgeBase articles.
Is the workbook Shared?
http://support.microsoft.com/kb/291287/en-us "Links in shared workbook return #REF! error in Excel"
Was the workbook last saved in an earlier version of Excel?
http://support.microsoft.com/kb/925893/en-us "External links may be updated in Excel 2007 when you open a workbook that was last saved in an earlier version of Excel"
Brad