All indications are that FCP7 and earlier will be EOL soon enough so whatever you buy pre-FCPX will not be supported by the OS in the future. I really hope I am wrong about this.
I have already decided not to go looking for a copy of FCP 7 and instead use the saved cash to upgrade my Adobe suite from CS5 to 5.5. I already have Premier in that package although I have not used it. Guess it's better to upgrade Premier and give it a shot than spend perfectly good money on software that may soon be unusable on Mac's running Lion or later.
I saw a few posts around the internet that FCS 2 runs under Lion. You have to install Lion over Snow Leopard as an upgrade instead of making a clean installation. Final Cut Studio 2 itself doesn't use Rosetta, it's just it's installer that does. I'm hoping it's true because I never upgraded past FCP 6 either.