Hansjörg,

You would have to get the UIElement from the current mouse position and check to see if you can get a tab element, then mark it as selected if it is not already.  The following code should cover this:

private void ultraTabControl1_DragOver(object sender, DragEventArgs e)
{
    Point clientPt = this.ultraTabControl1.PointToClient(new Point(e.X, e.Y));
    TabItemUIElement tabItemElement = this.ultraTabControl1.UIElement.ElementFromPoint(clientPt) as TabItemUIElement;
    if (tabItemElement != null)
    {
        UltraTab tab = (UltraTab)tabItemElement.TabItem;
        if (!tab.Selected)
            tab.Selected = true;                   
    }
}

-Matt