I want to be able to create a fade animation on a text element when the binding to that element updates. In other words, the effect is that as text gets added to a text box
You can set properties on your binding that will cause events to be fired. There are two properties available:
From the description of your setup, it sounds like you want to use NotifyOnTargetUpdated. Set that to true in your binding, then whenever the target updates, the Binding.TargetUpdated event will fire. You can then listen to that event in an EventTrigger, and fade your text.