In late September Microsoft admitted there was a bug in Excel 2007 which caused the calculation results of certain values to be displayed incorrectly. The results were correct, but the display was not. Microsoft has posted an update on their Excel blog, and there's a fix available. However, you'll have to download them manually for now.
We are in the process of adding this fix to Microsoft Update so that it
will get automatically pushed to users running Excel 2007 or Excel
Services 2007. Additionally, the fix will also be contained in the
first service pack of Office 2007 when it is released (the release date
for SP1 of Office 2007 has not been finalized).
Now, if they would only patch / fix / update Office and give us the option of getting rid of the ribbon ...