Here are a couple developer focused articles on Excel Services that were published today.
Summary: Optimize version 1 of the Excel Services programmability framework to extend Microsoft Office Excel 2007 and Excel Services functionalities to work with Microsoft Office SharePoint Server 2007 lists, query tables on Excel Services, external workbook references, SQL write-back, and user-defined functions (UDFs). (19 printed pages)
Summary: Learn how to develop and reuse user-defined functions (UDFs) for use in Microsoft Office Excel 2007 and Excel Services in Microsoft Office SharePoint Server 2007. (13 printed pages)