I only noticed this Google Spreadsheet feature today. But, it may have been available for a while. If you use a Windows Mobile Pocket PC or Smartphone with Google Spreadsheet, it gives you the option to download an Excel XLS file. This worked fine for me. The XLS spreadsheet opened up in Excel Mobile right after the download finished.
This is a great feature for me. It means that I can frequently used spreadsheets in Google Docs and download them to a Pocket PC or Smartphone when I want to work with them offline. Unfortunately, you cannot email the spreadsheet back to Google Docs as you can with text documents. I’ll guess this feature will appear someday though.