There is no backward compatibility. CS4 projects can not be savecd in CS3 format, nor opened in CS3.
Just in case my ID was not recognizable (Jive screwed up again): NO, there is no backward compatibility. There never was and there never will be.
What ?!?!?! Really? Oh glad I asked then!!!
I cant find Premiere Pro CS3 for sale though, thats not so good. Bit concerned about buying from third party concerning license etc.
I am a bit disappointed with that.