In his previous post, Marlon Grech explained how Silverlight 4 lacks the UpdateSourceTrigger for PropertyChanged and how to overcome this issue for the TextBox. In this post he shows how to do this for any Dependency Property of any Framework element.
Source: C# Disciples
Please note that this post is quite an experimental one. Probably the use case you are looking for is for the textbox scenario, if that is the case I would suggest that you use the more explicit approach i.e the one I show in my previous post. The take away from this post should be more the idea of how things work rather than the actual code I am using here, this code was never tested in production thus it might contain memory leaks and other issues.