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If you want to open the file in CS5, all of the linked files must also be CS5 or earlier. I ran into this, myself, a few months ago, trying to backport a file for a cleint I had mistakenly created in the wrong version.
I don't recall for sure whether I had to relink converted files in CS5.5 before making the .idml, or if I was able to set, in my case CS4, not to check links on file open so that I could open the CS5.5 idml and then relink to previously converted versions of the the links. Either way, though, you have to convert ALL of the files used, it just a question of wheter you'll need to relink in CS5.5 or in CS5.
As far as going back to CS5.5, that's not a problem, but INDD2 will not reconvert to the new version on its own. If there are no changes to that file other than conversion you can simply relink to the old unconverted version when you move back.
Thanks for the input Peter. Especially this:
"If there are no changes to that file other than conversion you can simply relink to the old unconverted version when you move back."...I wish I tought of that, so simple yet efficient