In this post Damian Schenkelman compares the validation provided by the RI to the one that comes with Silverlight 3.
I came up to the conclusion that the approach provided by p&p is pretty similar to one of the approaches Silverlight 3 allows related to validation. You can watch this video about Silverlight 3 Validation which explains some of the possibilities you have.
After that I decided to replace the RI’s validation using Silverlight 3 and was able to do it with some minor changes (you can check the OrdersDetailsView and OrdersDetailsPresentationModel which have most of the changes).