This will enable submit once all fields are integers, but it'll need more added to it to handle cases where the values could get changed back to non-integer after first passing the validation.
function doGet() {
var app = UiApp.createApplication();
var field1 = app.createTextBox();
var field2 = app.createTextBox();
var field3 = app.createTextBox();
var submit = app.createButton('SUBMIT').setEnabled(false);
var handler = app.createClientHandler().validateInteger(field1)
.validateInteger(field2).validateInteger(field3)
.forTargets(submit).setEnabled(true);
field1.addValueChangeHandler(handler);
field2.addValueChangeHandler(handler);
field3.addValueChangeHandler(handler);
app.add(field1).add(field2).add(field3).add(submit);
return app;
}
Corey's UiApp posts are helpful in this area.