Excel Services 2007 does not allow you to "save" back to the document library - all changes you make are transient in nature - they are gone. You can open in Excel and then save and you can use other tools to allow you to save, but you can't do it from the EWA proper.
Shahar Prish
Blog: http://blogs.msdn.com/CumGranoSalis
Professional Excel Services: http://www.amazon.com/Professional-Excel-Services-Programmer/dp/0470104864