You will be able to validate data in Silverlight using the Silverlight 3.0 validation feature. This project explains how to create a Extension class that would display a custom validation error on TextBox controls.

The live version on this project can be viewed here: http://silverlighttips.com/?tag=/validation.

Related topics:
Silverlight QueryString Validating ID, GUID and Timestamp (Date)
Extension methods in Silverlight and C#