Happens both on Mac and Windows with promoted build JavaFx 2.1 b05.
I didn't see this behavior before - although it might have been there.

First - when you drop a text area from the SceneBuilder library, the text is drawn on two lines, which is surprising since there's room enough to draw it on 1 line - see textarea1.png attached.

Then, if you anchor the text area on 4 sides, save the FXML, and preview the saved fxml, you will see that making the window too small will cause the text and scrollbar to display outside of the textarea, see attached textarea2.png

For reference I have also attached the textarea.fxml file: you can use the FXML loader to load it and reproduce step 2.