There is meant to be a converter for Project 2003 to read '07 files, but I
couldn't find it after a quick search of Microsoft.com. Until it is
available, you need the owner of the '07 file to save as to 2003 format.
I downloaded a beta of the compatibility addins a while back. I'm not
certain they're still available for download. The catch is that you can't
install or use the beta version if you also want to install one or more of
the Office 2007 applications.
Has anyone experienced problems with saving in 2000-2003 format from 2007 and
then when someone opens the file in MS project 2003, the files are not quite
right. Some data corruption ensues?