Your situation sounds strangely familiar :)... My opinion is that if I am going to design a form that is going to run in a JavaScript app, I should be able to do so in pure JavaScript. For example:
form = new UberUI.form();
form.append( new UberUI.input( {type: "text", name: "login", label: "Login:", validator: myValidatorFunction} ) );
...
form.show();
On a different note, jQueryUI is much more theme'able than ExtJS, which is great.
