Erika is working on pulling together more coding examples for the OpenXML formats. We've seen a bunch of stuff around WordprocessingML, and now they are starting to work on some good examples with SpreadsheetML and PresentationML. So if you've ever wanted to build processes that consumed or generated Excel files, here a great starting point: http://blogs.msdn.com/erikaehrli/archive/2007/03/08/VisualHowTosExcel2007TheOfficeOpenXmlFormats.aspx
We've had folks asking for years if there could be an easier way to work against Excel files outside of Excel (for server side processing, etc.). The earlier version of the SpreadsheetML format introduced in Office XP was a great first step, but now with the new SpreadsheetML that's part of the OpenXML formats you have complete control.