From Outlook Help:
"Symptoms
When you attempt to import information from a Microsoft Office Excel 2007
worksheet, Microsoft Office Outlook 2007 fails to recognize the file.
Cause
By default, Office Excel 2007 uses a new file format that has file
extensions of .xlsx, .xlsm, or .xlsb. Office Outlook 2007 does not support
opening files in these file formats.
Resolution
Office Outlook 2007 supports the previous Excel file format, which uses the
file extension .xls. You can save your Office Excel 2007 worksheet in the
file format used by previous releases of Excel and then import data from that
file into Outlook."
I'm trying to import an Excel 2007 file into Outlook. Having to retro my
Excel file to 97-2003 seems a step backwards.