After a search for using XSLT to produce Microsoft Excel spreadsheets I found this excellent blog by Brian Jones a program manager in Office. I was looking for solutions to a bug one of our customers encountered with using calculations in the Excel files. I thought there was a difference in the number format as he was using a Norwegian version of Excel, but turns out it was Excel 2000 and you need to have Excel XP or 2003 with Excel files created with SpreadsheetML.