While developing Offline Web Browser for Windows Phone i had to build a small MVVM framework in order to keep the logic out of the views. One of the first thing i found missing was a way to force immediate propagation of text entered in TextBox control to the databound property of my ViewModel. By default, TextBox binding is triggered only when control loses focus, and this is kind of lame.
I also felt that this needs to be solved and built into the wp7 platform but it seems that they were maintaining compatibility and also 'features' and bugs :)
Anyhow at least the Rx is included there.
And writing Behaviors is fun and you get the reusable behavior so I'm ok doing it once.