You might be fixing this and it might be early to post about silverlight 5 things, but when I open up a new window in OOB and try to use the drag and drop group by feature of the grid view it doesn't work. I can't see the mouse icon dragging the column. But if I move the mouse while dragging, over the main window I can see the icon.
I'm using the new seperate Single Border Window feature in Silverlight 5 and I'm putting my gridview in the new window
Window wd =
WindowStyle = WindowStyle.SingleBorderWindow,
Width = 800,
Visibility = Visibility.Visible,