Back when CS2 first came out, I was using CS at home but had CS2 at School/work. There was a plug-in that I downloaded from Adobe to back save. Haven't seen one for CS3 though but check around Adobe's site.
I just ran into this issue - you save it as an .inx file, and as long as your copy of CS2 is up-to-date, it will open with few or no issues...it's odd that Adobe hid this "feature" in this manner though.
If you are saving from CS3 to CS2 you must export the file as an InDesign interchange file. CS2 must also be updated to version 4.0.5(free update) in order to open the exchange file. Also if that CS3 file is going to be sent to a CS user, CS2 must first open interchange file and then re-export the file again to an interchange file again.
If you do any CS3 only effects (such as transparency effects) when you open the interchange file in CS2 they will not be present since CS2 does not have the abilility to create those effects. Also please be aware of possible text reflow issues since Adobe tweaked the text engine between CS3 and CS2.