Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.
http://www.hardcodet.net/2009/01/combinding-wpf-validation-rules-and-idataerrorinfo-to-validate-conversion-errors
Related:
http://karlshifflett.wordpress.com/mvvm/input-validation-ui-exceptions-model-validation-errors/
http://www.hardcodet.net/2008/04/wpf-custom-binding-class
http://joshsmithonwpf.wordpress.com/2008/11/14/using-a-viewmodel-to-provide-meaningful-validation-error-messages/
http://blogs.msdn.com/wpfsdk/archive/2007/01/16/someone-s-been-sitting-in-my-chair-property-changed-detection-mechanisms-in-wpf.aspx