I have noticed some differences between IE6 and Firefox (2 or 3RC1) on Windows XP in margins/height sizing. I have well declared the HTML 4.01 Transitional DOCTYPE in my HTML file. It looks like a min-height issue but I'm not sure. I put few code lines showing the issue.

- first, with a form panel, the panel content is empty on FF, some blank space on IE :
     public void onModuleLoad() {
        final Viewport viewport = new Viewport();
        viewport.setLayout(new CenterLayout());
        final FormPanel formPanel = new FormPanel();
        formPanel.setHeading("My panel");
- second, for a menu bar with border layout : on FF, the bar get the asked height/margins, not on IE
    public void onModuleLoad() {
        Viewport viewport = new Viewport();
        viewport.setLayout(new BorderLayout());

        ContentPanel mainPanel = new ContentPanel();

        BorderLayoutData mainCenter = new BorderLayoutData(
        mainCenter.setMargins(new Margins(0, 5, 5, 5));
        viewport.add(mainPanel, mainCenter);

        ToolBar topPanel = new ToolBar();
        TextToolItem tool1 = new TextToolItem("File");
        BorderLayoutData northData = new BorderLayoutData(
                Style.LayoutRegion.NORTH, 21); //no issue with a 30 pixels or more height
        northData.setMargins(new Margins(5));
        viewport.add(topPanel, northData);

        ContentPanel west = new ContentPanel();
        west.setHeading("My application");

        BorderLayoutData westData = new BorderLayoutData(
                Style.LayoutRegion.WEST, 200, 150, 350);
        westData.setMargins(new Margins(0, 0, 5, 5));
        viewport.add(west, westData);