During a recent POC, I ran into this with a customer and also had the question come up several times so I thought I’d post the answer:
Q: Why can’t I “Publish to Excel Services” from Excel 2007? The option is not available.
A: The following SKUs of Office 2007 contain the “Publish to Excel Services” capability: Ultimate, Professional Plus, and Enterprise. The Professional SKU does not have this feature. The different SKUs can be viewed here. Even without this feature, you can still save your Excel 2007 spreadsheet to a MOSS document library and render the workbook using Excel Services. However, what you’ll miss will be the Excel Services options that allow you to specify parameters and set which worksheets and/or charts are viewable by the user in the browser.