ViewStateUserKey has been around for many years and is an easy solution to prevent the infamous XSRF or cross-site request forgery class of attack.

It's documented:

http://msdn2.microsoft.com/en-us/library/system.web.ui.page.viewstateuserkey.aspx

ViewStateUserKey mitigates XSRF by including a unique identifier in the user's request.

This protection mechanism has been available for many years when Microsoft identified the one-click attack, now more commonly referred to as XSRF.