There is a known issue with array formulas that were deleted that was fixed in this update
http://support.microsoft.com/kb/958267 "Description of the Microsoft Office 2008 for Mac 12.1.3 Update"
Do you have iWork installed on your computer? The Numbers application has been reported a number of times in web forums as being able to open .xlsx files that were reported as corrupted. You can then save the workbook as a .xlsx file using Numbers, and it will open successfully.
On a Windows machine, this type of error occurs when I save the file in .xls format with a .xlsx extension (or vice-versa). In such cases, changing the file extension will allow the file to be opened successfully.
Brad
