Anyone who releases versioned software can take a hint from Adobe, and force upgrades on your users by making the backwards compatibility next to impossible:

Note: To open the InDesign CS5 document in InDesign CS3, open the exported IDML file in InDesign CS4, save it and export it to InDesign CS3 Interchange (INX), and then open the exported INX file in InDesign CS3. Make sure that all InDesign versions are up to date.

via Save backwards to the previous InDesign version

You have got to be kidding me! If you need to go to CS3, why would you have CS4 still hanging around?

Note: I realize CS3 is two versions back now, but going from InDesign to any previous version is difficult even if going just one version back. I plan to upgrade to CS5, but never went to CS4 because the many of the new features were ‘meh’ in my opinion and didn’t warrant the cost.