Use the following steps:
Export to PDF from CS5 using the Press Quality export setting.
Open that PDF in CS3 using PDF2ID.
OK it's a pretty crappy Indesign document, no offence Recosoft, but it was using the old PDF2ID plugin of which the new one is apparently much better, and given the feature difference between CS5 and CS3 it was never going to be that great. So to all those that say that they would pay money for a plugin to do it instead of paying for an upgrade which at least keeps you current, here's your option. For extra versatility up your resolution settings in the PDF export so your links will be higher resolution (note - I haven't tested to see whether this works).
I was wanting to do this and it got me thinking of all the angst that has been vented on this question; how Adobe should make it possible, how it's a conspiracy byAdobe to force us to upgrade, Illustrator and Photoshop can do it etc etc. (BTW note the contradiction of those last two statements!) And I thought about this article here from the extremely knowledgeable Harbs and nearly gave up. But then I remembered I had an old version of PDF2ID installed with CS3 and it was a simple layout so went ahead and did it no problem. If you had an even simpler document and a little bit more time you could do it without the PDF2ID plugin but use Illustrator instead which I tried to do initially. However unless I'm doing something wrong you can't do a whole page at a time but have to do elements bit by bit, not to mention the horrible way text gets copied.
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...