In Excel 2003 you were able to sync amendments both from a Excel 2003 spreadsheet to a SharePoint List and from a SharePoint List to an Excel 2003 spreadsheet. But in Excel 2007 this is no longer the case... Well it was until Microsoft released an Excel 2007 Add-in. This is a must install if you are upgrading to Office 2007 from Office 2003, otherwise your users will cry.
The Excel 2007 Add-in, can be downloaded from: http://www.microsoft.com/downloads/details.aspx?FamilyID=25836e52-1892-4e17-ac08-5df13cfc5295&DisplayLang=en
There is also an accompaning MSDN article "Publishing and Synchronizing Excel 2007 Tables to SharePoint Lists": http://msdn2.microsoft.com/en-us/library/bb462636(office.11).aspx
Creating a SharePoint list from within Excel 2007 is known as exporting an Excel table. In Excel 2003, this was known as publishing an Excel list. Once the table data is placed on the SharePoint site, users can see the Excel 2007 data without having to opening Excel 2007. You can maintain a link between the SharePoint list and the Excel 2007 data, but without the Excel 2007 Add-in, any changes in the Excel 2007 spreadsheet are not reflected in the SharePoint list. Without the Excel 2007 Add-in, you can only synchronize changes in the SharePoint list to the Excel 2007 spreadsheet - but you cannot save the workbook in the new Office Open XML Formats. Instead, to retain the functionality, you need to save the workbook in the Excel 97-2003 file format.