Exception Details: System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that machineKey configuration specifies the same validationKey -- when navigating the Admin UI

After upgrading to SP10, users receive this error when taking various actions throughout the admin UI such as:

--clicking on logout link
--creating a new part
--saving a form.
--editing an existing part

This often occurs if a user is logged out and needs to log back in.