Hello,
I am getting almost the same error as a previous poster here -but with Q3 2009 (however it worked fine with Q2 2009). -the login.aspx is the first form that gets loaded and its throws a "Sys is undefined" javascript error -but despite this error, I am able to login and continue. All subsequent pages that use the FormDecorator control work fine, it just seems to be that first page (login.apsx).
Cheers.