Reply To: Can I still use CS3 after upgrading to CS5?

If you are going to thrash that old PPC Mac, yes, it'd be wise to de-activate the old ID. You can re-install your CS3 a couple of times, but I lost count of how many times I did that and forgot (or just couldn't) de-activate it first. So next time, it's gonna be a gamble if it still works.

There are a few known issues with running different versions side-by-side: you must install them in chronological order (don't first install CS5 then CS3), and your system may attempt to open a double-clicked document in “the wrong version”. The latter is not a problem if it attempts to open a CS5 document into CS3 (it will simply fail) but the other way around, you'll be inadvertently 'upgrade' a document to CS5. And there is no way back from CS5 to CS3, without a CS4 to sit inbetween.

As for the operating system for new iMacs: ask Apple … (it's a fair bet they won't be doing that.)