I have just written a new article over at www.codeproject.com which talks about how to perform cross business object validation using a new idea that I had the other day. This technique is quite different from the standard IDataErrorInfo approach. The idea is that Each View is bound to a ViewModel, where the ViewModel may hold many business objects which may need some sort of cross validation.
As such each ViewModel is validated by a independent validator. You may want to read more about it over at http://www.codeproject.com/KB/WPF/GlobalWPFValidation.aspx