Personally, I use Excel 2003 at work and Excel 2007 at home. I am constantly (as in 3 or 4 times a weeks) using Excel 2003 files in Excel 2007.
I found this tip at vista4beginners.com showing how to have Office 2007 automatically have files in the Office 2003 format: Configure Microsoft Office 2007 to save files using the old Office 97-2003 format