They're allowed in products where you would be allowed to use milk or milk products in the composition of a product. Now, depending on the product, you may not want protein in it. For example, there's not much protein in ice cream, so milk protein concentrates wouldn't be used in that. But for most of the products that allow milk or milk products, if MPCs would be useful, they could be permitted.
On June 1st, 2006. See this statement in context.