Hi Peter,
Based on my understanding the cause of this behavior could be that your DataGrid is being wrapped inside a StackPanel and the DataGrid's Height property has not a fixed value and as this panel will not limit the height of your control your vertical scroll bar will not be shown. This can be seen in any WPF application when defining a Datagrid inside a StackPanel.
A possible scenario where this could also happen, is if your region is defined in an ItemsControl which by default defines the panel to use for the layout of its items as a StackPanel. Hence in order to avoid this kind of problems you should have to replace your layout panel with a different panel which limits the height of it's children, or set a fixed value for your DataGrid control.
Also, in my experience this problem should not happen when defining regions in controls like a ContentControl or TabControl, as they do not use a StackPanel as their layout panel.
I hope you find this handy
Agustin Adami
http://blogs.southworks.net/aadami,