First, just to check the simplest solution, you aren't posting back "old" pages are you? By this I mean getting the page when the viewstateEncryptionMode is off/auto, switching the mode to on then posting back. This wide cause the error, but is unlikely Tokyo are doing this.
This problem is ringing a bell, but I can't place my finger on it. You might be suffering from this http://blogs.msdn.com/b/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx (On my mobile so can't make clickety),though this was fixed IIRC.