Pavan has a nice post on creating baloon comments. The first thing that comes to mind on seeing the balloon comments is Comics. And guess what! thats exactly what I tried.. and presto
You can get the original source code here. It overlooks positioning the arrows coming out of the comment boxes - this is a minor detail and I added a dependency property for this purpose. The update code is attached.
As for the usage: <custom:BalloonDecorator Background="Pink" PointerLength="30" CornerRadius="5" CornerPosition="Right" .... />