There is a converter for docx files to make them readable in 2004 available from MS -
HERE
Its a converter not a plugin for Office so I think you have to drop your docx onto the converter prog, then open it in office - also its a beta release. Some 2007 features may be lost I guess. Office 2008 for mac is supposed to be out in Jan 2008 and will also use the xml format so will be natively compatible with 2007.