Sorry, I drifted into Instant Message language...
AFAIK = "As far as I know"
Simply stated... back saving to earlier version formats can't be done.
Look at:
http://www.adobe.com/support/techdocs/322be.htm
Note also, that while the InDesign CS Help file implies that is can be done, the "InDesign CS ReadMe.pdf" on the CD or in your InDesign main folder retracts this claim.
I don't know of any workaround if the receiving user needs to be able to edit the document.
Richard
Richard in Rochester