Disabling submit button until all fields have values

$('#user_input, #pass_input, #v_pass_input, #email').bind('keyup', function() {
    if(allFilled()) $('#register').removeAttr('disabled');
});

function allFilled() {
    var filled = true;
    $('body input').each(function() {
        if($(this).val() == '') filled = false;
    });
    return filled;
}

JSFiddle with your code, works :)

Check out this jsfiddle.

HTML

// note the change... I set the disabled property right away
<input type="submit" id="register" value="Register" disabled="disabled" />

JavaScript

(function() {
    $('form > input').keyup(function() {

        var empty = false;
        $('form > input').each(function() {
            if ($(this).val() == '') {
                empty = true;
            }
        });

        if (empty) {
            $('#register').attr('disabled', 'disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
        } else {
            $('#register').removeAttr('disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
        }
    });
})()

The nice thing about this is that it doesn't matter how many input fields you have in your form, it will always keep the button disabled if there is at least 1 that is empty. It also checks emptiness on the .keyup() which I think makes it more convenient for usability.

For all solutions instead of ".keyup" ".change" should be used or else the submit button wont be disabled when someone just selects data stored in cookies for any of the text fields.

All variables are cached so the loop and keyup event doesn't have to create a jQuery object everytime it runs.

var $input = $('input:text'),
    $register = $('#register');    
$register.attr('disabled', true);

$input.keyup(function() {
    var trigger = false;
    $input.each(function() {
        if (!$(this).val()) {
            trigger = true;
        }
    });
    trigger ? $register.attr('disabled', true) : $register.removeAttr('disabled');
});

Check working example at http://jsfiddle.net/DKNhx/3/