No guarantees, but I did have success working in FileMaker Pro 6 with a copy of a client’s data in Leopard. I would not try this with live data at all. But, it’s nice that I did have no problems working in FileMaker 6 for 30 minutes doing several different operations.
FileMaker Inc. will not go back and test FileMaker Pro 6 to assure that it is fully compatible with Leopard. They don’t have the resources to go back that far. That means, use at your own risk. This is a tough situation for FileMaker consultants because most of us want to run the latest version of OS X. I have enough computers (3) that I can keep one on Tiger, if need be, but many of us will need to choose to stay compatible or move on.