It seems that by default, SharePoint lists linked to Excel only sync one way if you are using Excel 2007. If you are using Excel 2003, they sync two-way.
Reason for this? Access 2007 is now the favorite “off line client” for SharePoint lists.
If you install an add-in for Excel 2007, lists can again be synced two-way.
See: http://msdn.microsoft.com/en-us/library/bb462636(office.11).aspx