According to Eric Gunnerson we now write Static classes in C#

So, for Whidbey, we allow the user to mark a class as static, which means that it's sealed, has no constructor, and the compiler will give you an error if you write an instance method.

Rumor has it that the 1.0 frameworks shipped with an instance method on a static class.

This would be nice to have in the language. I know it would have helped us catch the fact that the System.Xml.XmlConvert class which only has static methods was shipped with a [useless] default constructor.