Marlon Grech explains how you can specify that a certain binding should update the source on PropertyChanged in Silverlight 4.
Source: C# Disciples
In Silverlight 4 there is no way out of the box to specify that a certain binding should update the source on PropertyChanged, the options for the UpdateSourceTrigger are LostFocus and Explicit.
These options are more or less what you really need for most scenarios, yet sometimes you want to have the ability to update the binding while a property is changing. A use case would be, you have a textbox and you want to apply some validations while the user is typing, maybe you enable a button if the text entered is correct or disable it if it is not correct.