One way to build a unit-testable jQuery plugin.

testablePlugin.js

var myWidgetCreator = (function ($, lib) {

    var Creator = {};
    
    Creator.create = function (args) {
        // 'this' is the captured jQuery object
        // because we called 'apply' from the widget
        this.html(Creator.doSomething());
    };
    
    Creator.doSomething = function () {
        return "WIETERS";
    };

    return Creator;
    
})(jQuery, {});

jQuery.fn.myWidget = function (options) {
    myWidgetCreator.create.apply(this, options);
};

jQuery(document).ready(function ($) {
    $("body").myWidget();
});

With this method, you can only test methods in myWidgetCreator that don't use this, but any methods that use this should be tested via the plugin itself. Something like Creator.doSomething could be unit tested using this method, though.

Actually, now that I think about it... you can test any of the methods that use 'this' and assume it to be a jQuery object just by passing it a dummy jQuery object like so:

myWidgetCreator.create.apply($("<div>"), options);