Illustrator CS5 can save in older formats well below CS4: When saving or doing a Save As for the first time, you can choose Illustrator CS5 (15), CS4 (14), CS3 (13), CS2 (12), CS (11), 10, 9, or earlier.

Note that this is not the case for Adobe InDesign. InDesign CS5 (7) maintains backward compatibility only to CS4 (6), and only when the INDD document is exported in IDML (InDesign Markup). Similarly, InDesign CS4 (6) is backward-compatible with CS3 (5) when exporting to INX (InDesign Interchange).

--Gerrit

Well, the point of this hints, really, is more about using a newer (postscript-EPS format) file and be able to use it with a legacy software, not really about "how to save a file to a legacy format"...