Rockford Lhotka raises an interesting point about System.ComponentModel.DataAnnotations missing from .NET FX4.0 ClientProfile:
http://www.lhotka.net/weblog/DataAnnotationsNotInTheClientProfile.aspx
Being a big fan of DataAnnotations (finally a validation engine shared between WPF/SL/RIA Services) I invite you to vote here:
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=508846
Corrado