Ran across this error trying to get a datalist from the old CW site working under 2.0. I've seen some of this with third party controls but setting the page directives to:

enableEventValidation="false" viewStateEncryptionMode="Never"

worked for me...

Found the answer here: http://dotnetjunkies.com/WebLog/thomasswilliams/archive/2006/06/06/139890.aspx