Microsoft has always supported older document formats. For example you can open a Word or Excel docment created in a previous version of these in Office 2007. The exception to this has always been Access. Many features that are used in Access databases have had to be converted over to the new version. Sometimes that worked, some times it didn't.

The statement that you can't open Office 2007 documents in Office 2003 is not 100% correct either. You can open Word documents created in Word 2007 with 2003, but you can't open Excel 2007 (saved in it's native format xlsx) with Excel 2003. You would have to save it as an Excel 97-2003 version.