Up through CS4, 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)" ?>'
In CS5 the interchange format changes to IDML, which so far appears to have no such tricks. You have to downsave to IDML to open in CS4, and from there to INX, etc. If anyone knows of a converter or code hack, please post it.