To enable a round-trip from export to import without any file format modifications, Products and Web-apps export now generate files in raw CSV format so they can be directly re-imported after you make modifications to the data only. A couple of caveats to be aware of:

The extension of the generated file is still ".xls" so that Excel will be the default application used to open the export file. However, if you open the file using a text editor such as UltraEdit, you will see the data conforms to the CSV RFC meaning that linebreaks within fields are allowed if the field is enclosed with a double quote "".
When saving the file in Excel you need to explicitly choose the Save As '.csv' format to ensure that Excel doesn't convert the file into a proprietary Microsoft format which can't be imported.

These changes have vastly improved the performance and reliability of Product and Web-app items export functionality.