With the user-base of Office 2007 starting to grow, chances are one of these days you may receive a Word 2007 file that you can't open with your Word 2003 or earlier version. The same is true for Excel 2007 and PowerPoint 2007 files. This has already caused some problems for some of my clients so what I've been doing is after we install Office 2007, we immediately change the default settings to tell Office 2007 to save the files in Office 2003 format.
But just in case you happen to get one of those Office 2007 files (the file extensions are .docx for Word, .xlsx for Excel and .pptx for PowerPoint) the good news is Microsoft has an Office Compatibility Pack that enables older versions of Office (only up to Office 2000) to open and save in the Office 2007 format. It's an easy install and works very well. My only other wish is that Microsoft includes this as an automatic update so users don't have to separately install it.