What does the __VIEWSTATE hold?
Paul Wilson has a very good article: ViewState: All You Wanted to Know
VIEWSTATE can be deserialized with the LosFormatter class.
A quick Google search answers the question:
http://msdn.microsoft.com/en-us/library/ms972976.aspx
First sentence:
Microsoft® ASP.NET view state, in a nutshell, is the technique used by an ASP.NET Web page to persist changes to the state of a Web Form across postbacks.