Hi mjohn,
I can produce this problem using the VS web server but not in IIS so its really not a bug in production installations as far as I know. However the stange thing for me is even though I get this error in VS web server, when I view the source of the page I see the form as <form method="post" action="default.aspx" id="aspnetForm"> which should be fine but still the error happens. I do not know why it would render differently on your machine.
Best,
Joe