As you may have found out in unhappy circumstances, there is no such thing as a legacy format dropdown menu when saving an InDesign file. For those of us who've upgraded to CS3 everywhere, no problem right? However, if you're like me and only have access to CS3 at work or school and still have previous versions at home, there is a solution:

It's called the InDesign Interchange format, file extension .inx. All you have to do to save an InDesign CS3 file down for legacy versions is  go to File>Export and choose InDesign Interchange from the export file format dropdown menu.  These files take slightly longer for InDesign to read upon initially opening them, but other than that they operate perfectly.

One catch: InDesign CS or CS2 right out of the box cannot open .inx files, you need program updates that keep your version up to date. So if your .inx file doesn't work initially, try updating your program using the free updates available on Adobe.com. There's even an easy way to check. if your version number reads 4.0, you probably need an upgrade. 4.0.1 or later and you're golden :)

Hope that helps with (one of the many) inconveniences of CS3!