Code

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;
using System.Windows.Shapes;

using MindFusion.Diagramming.Silverlight;


namespace Test
{
	public class MyLink : DiagramLink
	{
		public MyLink(Diagram diagram) : base(diagram)
		{
		}

		public MyLink(Diagram d, DiagramNode origin, Point point) :
			base(d, origin, point)
		{
		}

		protected override void UpdateVisuals()
		{
			base.UpdateVisuals();

			var canvas = Content as Canvas;
			if (canvas != null)
			{
				if (!canvas.Children.Contains(baseTextBlock))
				{
					canvas.Children.Add(baseTextBlock);
					baseTextBlock.SetBinding(TextBlock.TextProperty,
						new Binding("BaseText") { Source = this });
				}

				const int margin = 5;
				var linkBounds = GetBounds();
				var points = ControlPoints;

				var measure = new TextBlock
				{
					Text = BaseText, FontFamily = FontFamily, FontSize = FontSize
				};
				measure.UpdateLayout();

				if (points[0].Y > points[1].Y)
				{
					Canvas.SetTop(baseTextBlock,
						points[0].Y - linkBounds.Y - margin - measure.ActualHeight);
				}
				else
				{
					Canvas.SetTop(baseTextBlock,
						points[0].Y - linkBounds.Y + margin);
				}

				if (points[0].X > points[1].X)
				{
					Canvas.SetLeft(baseTextBlock,
						points[0].X - linkBounds.X - margin - measure.ActualWidth);
				}
				else
				{
					Canvas.SetLeft(baseTextBlock,
						points[0].X - linkBounds.X + margin);
				}

			}
		}

		static public DependencyProperty BaseTextProperty = DependencyProperty.Register(
			"BaseText", typeof(string), typeof(MyLink), new PropertyMetadata("", OnTextChanged));

		private static void OnTextChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			var link = (MyLink)d;
			link.UpdateVisuals();
		}

		public string BaseText
		{
			get { return (string)GetValue(BaseTextProperty); }
			set { SetValue(BaseTextProperty, value); }
		}

		private readonly TextBlock baseTextBlock = new TextBlock();
	}
} 

Code

public class MyBehavior : LinkShapesBehavior
{
	public MyBehavior(Diagram diagram) : base(diagram) { }

	protected override DiagramLink CreateLink(DiagramNode origin, Point point)
	{
		return new MyLink(Diagram, origin, point);
	}
}