Because all functions in C# must live inside of a class, there are some clases - System.Math is a canonical example - that are merely collections of static methods. Since it's useless to create an instance of such a class, in current versions of C#, you can protect against this by creating a private constructor. The constructor can never be called, and therefore no instance can be created.
There are three issues with this approach:
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.