It actually also ran for indesign cs4(really surprised) but did not work in CS3.
Again after looking at a handful of PDF's from Adobe, it seems as if it shouldn't be working.
For a jsxbin to work in CS3, you need to compile it in ESTK 2 (The CS3 version).
Assuming you don't use any features of ExtendScript that are not supported in CS3 (like some parts of E4X), jsxbin files created for CS3 will work in CS3/4/5.
CS4 and CS5 jsxbin files are interchangeable.
Re. #2: It's saved in Binary form which is enough to stop the casual hacker...