So far I have been able to bring complete projects from a WIN platform over to MAC. Any older versions of premier have to be saved as CS3 before it can be opened by CS3 on the other OS. Don’t get rid of the windows box too quickly. There is a major bug while capturing. I am using a dual quad core MAC. There are either dropped frames or drift while capturing from a deck on the MAC version. Adobe says they are working on a patch, but it has been a month. It is a pain in the A**. Capture on the win os and move the whole folder over to MAC then start to edit.
Also, if you have files scattered around on differnt drives you are going to have to go back and forth across a network to re link them.
Sorry I can’t answer the CS3 – FCP6 part of the question. I have FCP5, but haven’t tied to open a project.
G5, 1.7, 4MB RAM, 30″cine, G4, 1.6 2MB RAM, Mbox, Neumann TLM-103, FCP HD 5. Pro Tools, Adobe Creative Suite, Reason 3.0, Macromedia Studio, ProAnimator, HVX200 with Firestore v4.0