If you stay in a mixed CS4/CS5 environment, I'd recommend a cheap utility from Rorohiko called Soxy. It's only $19 and it can distinguish which version of an Indesign, Illustrator, Photoshop, or even Quark file you have and open it with the appropriate version of the app. It's not exactly what you're looking for, but it is useful for preventing inadvertent opening in the higher version. Trying to open in the lower version is less of a problem because it won't open the file, but it's a bummer to open a CS4 file and save and realize you've just converted the file to CS5 by mistake.
Unfortunately, they haven't released a CS5-aware version yet. (Coming soon, they tell me.) I bought 2 copies of it (for two workstations) when I upgraded to CS4 because a lot of my clients were still in CS3 until VERY recently. I don't get any $$ from Rorohiko, but my experience using it with CS2/CS3/CS4 was good enough for me to recommend it. It comes with the typical 30-day free trial, so check that out when they release it.