Downsaving creates an Adobe Interchange file with an .inx extension. This is a simple xml file that actually does work with all versions of InDesign (at least down through CS2), but has a limiter Adobe put in to annoy us all. To fix an .inx file created with CS4 to work with CS2 (and 3):
Open the INX file with your favorite text editor (like TextMate or BBEdit) and change the version number. Replace line number 2 (which looks like this):
'<?aid style="33" type="document" DOMVersion="6.0" readerVersion="5.0" featureSet="257" product="6.0(352)" ?>'
- with this:
'<?aid style="33" type="document" DOMVersion="5.0" readerVersion="4.0" featureSet="257" product="5.0(662)" ?>'