I think what people really want is a way to say in InDesign CS5 - "Save for CS3" It gives a warning on the dangers etc. perhaps even a txt file of features not supported by CS3. Even for it to throw up a "Make PDF of this file first"
OR if CS3 had an update to allow opening of later files. Perhaps CS3 could make a PDF of this without opening it (I don't know the ins and outs?) and then it opens it you get a reference file, a txt file and all sorts to ponder through to fix up the CS3 file.
An alternative to your method would the InDesign to Quark and Quark to InDesign method - software from Markzware (I think that's how it works?) You convert the InDesign file to Quark. Then back to InDesign.
Yeah I sort of know that (about what people want) but I doubt it's gonna happen. My post was made fairly firmly tongue in cheek; there is a way to do it!
It would be interesting to compare the two methods of round tripping PDF2ID or using Markzware, this thing about PDF2ID though is it doesn't require that you also have Quark...