I think Interface overwrites the jQuery's animate() function with its own, which may have something to do with it. I'm not sure why it would have adverse affects when using jQuery 1.1.3.1 but not when using jQuery 1.1.2.
Were you using the most up-to-date version of Interface? I can't tell from the file as it is packed. Maybe there is a newer version of Interface in a SVN repository that works better with jQuery 1.1.3.1?
Still pondering... Joel Birch.
