Confusing = with IS in MDX calculations

This topic has been written about many times, perhaps even by me. However, it is a common pitfall, so I am writing about it again. See this msdn post for a recent example. Essentially, the pitfall is when an author wants to check if the current member of a dimension attribute is at a particular member. The incorrect MDX might look like this IIF(DimCustomer.CustomerClass.Currentmember = DimCustomer.CustomerClass.CustomerClass.&[Gold], Measures.Price*0.9,Measures.Price)