Two (2) new articles have been published centered on Excel Services programmability with accompanying sample solutions available in the Microsoft Download Center. The first article demonstrates how to extend List and query tables on Excel Services and includes content detailing external workbook references, SQL write-back and user-defined functions. The second article provides a more in-depth approach to developing UDF's for Excel Services.
Extending the Excel Services Programmability Framework
SharePoint Server 2007 Sample: Extending Excel Services Programmability Framework Samples
Developing User-Defined Functions for Excel 2007 and Excel Services
SharePoint 2007 Sample: User-Defined Functions for Excel 2007 and Excel Services Sample Solution