By design Excel 2007 and 2010 now opens .ods files, and quietly and deliberately destroys any formulas and replace them with their result value. 

This makes it very dangerous for an organization that has standardized on .odf, to allow a user with Microsoft Office 2007 or 2010 to access their files. 

The Excel user may not notice that the spreadsheet is not functioning, since it looks "normal", so he may enter different values in the document, and not understand that Excel is not preforming calculations. 

This we can not prevent, and it has to be granted the responsibility of the Excel user.  

However, if he saves the .ods it will render the spreadsheet unusable also to all the LibreOffice users. And they will now have the same risk of believeing that their .ods file is working.

To prevent this I would like to suggest the following feauture to LibreOffice Calc:

When opening a .ods document LO should examine content.xml for signs of the file beeing "sabotaged" by Excel. 

By testing I have found that such an ods will containg the text:

xmlns:msoxl="http://schemas.microsoft.com/office/excel/formula">

inside content.xml. 

This is a sign that the file has been saved by Excel, and is a strong indicator to the suspicion that the file contains no formulas. 

If the text above is found in a .ods LO should give a warning like : 

"This file has been edited by Excel and have lost it's formulas". 

Or something similar.

This may not be the best way to do it, but it would help on the way.