SB 2.0 will output FXML files which are syntactically compatible with FX 2.2/JDK 7.
You will be able to load these files with FXMLLoader on the FX 2.2/JDK 7 platform under the condition that you do not use any of the new features introduced in FX 8/JDK 8 -- i.e. do not use any of the new classes, or new properties, which mention "Since: JavaFX 8.0" in the API documentation.
Thank you for the answer. I'm anxiously awaiting the developer preview. What Mo was showing at JavaOne looks like a great enhancement between the custom node support and the API to use parts of the tool in an IDE.