- Open up a new document
- Set it to landscape via File -> Page Setup
- Go to File -> Save As
- Switch to the /Applications/Microsoft Office 2008/Office/Startup/Excel folder
- Set the filename to Workbook
- Uncheck the Append file extension checkbox
- Quit and restart Excel
Your workbooks should now be landscape by default.
You can find this in the Excel help. Don't use the Help menu item on the main Mac menu bar, Excel's help isn't available there. Instead click the Help button on the toolbar in an Excel document to bring up the Excel help system and look for the "Control how workbooks and sheets are created" document, available under Contents -> Customizing Excel.