[xAockd]

var form = $('#myForm'),
    login = $('#login'),
    pass = $('#pass'),
    submit = $('#button');
form.change(function(){
    if(login[0].value != '' && pass[0].value != ''){
        button.disabled = false;
    }else{
        button.disabled = true;
    }
})

http://jsfiddle.net/wa5BM/