[Dino Esposito] …Introduced with ASP.NET 1.1, ViewStateUserKey is a string property on the Page class that only few developers admit to be familiar with. Why? Let’s read what the documentation has to say about it.

void Page_Init (object sender, EventArgs e)   
   ViewStateUserKey = Session.SessionID;

There will be a few more that are familiar with that now :-)