You can also create a new BasePage Class that over rides the html Rendering to ensure the __EVENTVALIDATION and __VIEWSTATEENCRYPTED etc are rendered at the top of the page. This way the viewstate hidden fields are rendered first before any other controls. Therefore, it minimises the chances of the encrypted viewstate not being available. Take a look at the blog on this subject here: "http://blogs.msdn.com/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx"
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