Sorry you are having trouble. I do not have a new integrate MU site to test on.
Did you try to eliminate things like plugins or theme conflicts?
If you can let me log in to trouble shoot the form code, please contact me here:
Maybe another plugin is conflicting.
Look on the Admin - Plugins - menu.
Temporarily Disable (not uninstall) all your other plugins.
Does it work now? If yes, Activate the plugins one by one to determine
which one conflicts. Which plugin was causing it?
It could be the theme you are using is causing it...
Do this as a test:
In Admin, click on Appearance, Themes.
Temporarily change your theme to the "WordPress Default" theme, or "Twenty Ten" (default for WP 3.0).
It does not cause any harm to temporarily change the theme and change back.
Does it work properly now?
If it does then the theme you are using is the cause.