You can save back to CS5 from CS5.5, can you not? Otehrwise you will have to uninstall your CS5.5 and then reinstall it after you have installed CS5. The apps do exist separately, but you simply have to obey a certain install order. If you experience problems, you may need to run the Creative Suite Cleaner Tool.
It's hard to belive but "Save As" and "Export" are incapable of saving compatible INDDs, InDesign relies only on the IDML for compatibility between versions. But I'll try your tip. Thank you.
Moving this discussion to the InDesign forum.
Send the client the IDML. There is no way to save an earlier version INDD file.