I found this at MS. It looks like, as long as you've downloaded the 2003 viewer, you can just go to your Excel 2007 file in your window explorer and double click the file. You can't open it directly from the 2003 by selecting file then open. This link also includes the download for the compatiblility pack. So it looks like you won't have to save it down to 2003 after all - This is for viewer only. If anyone has the actual Excel 2003 software they won't be able to open the Excel 2007 file unless it's saved down - they'll have to download the 2003 viewer/compatibility pack also.
Excel Viewer 2003
You cannot click File and then click Open to open a 2007 file in the Excel 2003 Viewer. To open the file, you must locate the file in Windows Explorer, and then double-click the file that you want to view.
To download the compatibility pack from the Microsoft Download Center, visit the following Microsoft Web site:
I have Excel 2007 software so maybe someone that doesn't have Excel can download the view/comp. and check it out for you.
Hope this works.