Lots of large agencies and printers cannot afford to update their software every 16 months when Adobe decides to change a few things. I don't think it's that terrible that they have asked this of you.
It will be quite simple for you as well to do so.

With your file open, go to File -> Export -> Indesign Interchange file (under formats). This will save an .indx file. This will now open in CS3 for them (or any subsequent version below that to the original CS) Don't forget to package fonts and links if necessary.

nevermind!!! I figured it out! what a hassle tho eh?

Well, I don't have CS4 yet, but if it's anything like 3, you should be able to got to File > Export > then select InDesign Interchange as the format. That should work if you haven't used any new for CS4 features. You'll still want to keep a close eye on your proofs to make sure it translates.

I was nice to them, and didn't say anything about it, just got the file to 'em. Thanks!!!

I don't know what program you're working in, but usually if you choose "Save As" it'll have some drop-down box where you specify what version. Sometimes the drop-down appears in a dialog box that appears after a first one.

I don't know if I've ever been asked by a printer to save down, but I have been asked by certain magazines. I usually don't care as long as it doesn't eliminate any features in the newer version. Most printers receive files by PDF nowadays anyway.

InDesign CS4 doesn't have a version save. Save As will give you either a CS4 document or a CS4 template. It has to be Exported an INX file (indesign interchange)

If you ever come across having to save down from CS4 to CS2 then you've really got your work cut out for you, as CS4 INX files are CS3 compatible only. Then you've got to open it in CS3, and down save to CS2. OR....you can cheat. I've learned how to save from CS4 to CS2 without the in between CS3.

Now, I will admit, that this way is a little sketchy and risky, but if you really need it and don't have CS3, it works (I sent some files to another designer once who was still in CS2. That's how I learned this trick).

in CS4, Export to your INX file.
Open your INX file in your basic text editor (I use Text edit)
You'll get pages of code. This is the XML code.
At the top you'll see the following:

<?aid style=”33″ type=”document” DOMVersion=”6.0″ readerVersion=”5.0″ featureSet=”257″ product=”6.0(352)” ?>

Change the DOMVersion and the readerVersion down 1 number each to get the following (realistically, you only need to change the readerVersion but I prefer the safety of changing both) :

<?aid style=”33″ type=”document” DOMVersion=”5.0″ readerVersion=”4.0″ featureSet=”257″ product=”5.0(662)” ?>

Now save the file. Do not change the extension. Keep it as an INX file!

Now it will open in CS2, without having to run it through CS3 first.

Just a tip for future use if anyone needs it.

Well well... lets just say not everyone is affordable to change or upgrade their software. At least they are not asking for CS1... my client's printer drives me nuts asking me for Illustrator 9.0... want to faint when I heard their request. Now that is horrible!

Just be careful downsaving, Adobe updates the type engine each version, so text may reflow and become overflowed in text frames. And newer functions such as cross-referencing, page transitions, etc. won't work anymore, especially if you go back as far as CS2, where there are no variables, auto numbering or things like that.

So make sure you keep a backup of the original file, just in case.

Here's the site that I learned the XML trick from:

http://indesignsecrets.com/double-downsave.php

forgive the print newbie but couldnt one just export as PDF?

If the printer is sign shop, I don't find that particularly horrific. It may be a function of rip support for the machinery he is using. I still have one vendor (but not a printer) that requests Illy v3 files cuz that is the only type of Illy file his engraving CNC will recognize.
<shrug>
My older Signlab only takes Illy v8. The newer one will take up to CS2. It isn't so much my lack of upgrading as the software company's inability to keep up with Adobe's 18 month schedule either. I'd never have you send those files though. I keep the main software up to date and dumb things down to suit me.

Generally whatever the printer has asked for is the best to supply them with that.
My printers almost all request Press-ready PDF's and they give me the export settings for them. But I have a few smaller ones still that want CS2 files, native with links etc.
Also depends on what you are printing... for vinyl cuts I always have to supply my Illy files.

When I was in the print shop, our vinyl cutting software could only support Illy v.8, so we'd have to down save all the files before it could cut them. Seems to be pretty common with vinyl to use v.8

yep. There are no transparency effects in V8. LOL!