Type 'Microsoft.SharePoint.SPUser' in Assembly 'Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' is not marked as serializable. Type 'Microsoft.SharePoint.SPUser' in Assembly

Make sure you are not storing any object of SPUser type in Session on ViewState. I was getting this error SPUser was persist in ViewState. We changed the logic to store LoginName and everything worked fine