Edit in JSFiddle

(function($) {
    $(document).bind('keydown keyup', function(e) { // capture all key up/down events
        if (e.keyCode === 91 || e.keyCode === 224) { // 91 is command key
            if (e.type === 'keydown')                // don't know why Firefox thinks it's 224
            $('body').addClass('cmdDown');           // and I'm too lazy to Google right now
            else $('body').removeClass('cmdDown');
        }
    });
})(jQuery);

<div id="wrapper">
        <h1>Magic cursor refresh</h1>
        <h2>(not really magical)</h2>
        <ol>
            <li>Click here to focus the page</li>
            <li>Hold your mouse still</li>
            <li>Hold down ⌘</li>
        </ol>

        <p>Hold your mouse still and press and release the Command key to see the cursor change in place. No, it's not a big deal. That's why this page is so short.</p>

        <p>If you're not seeing the effect, click on the headline to make sure the page is focused and capturing your keystrokes.</p>

        <p>Works best in Safari and Chrome. Kinda good in Firefox. IE? Dunno.</p>
    </div>

    /* boring stuff */
    body {
        font-family: "Helvetica Neue", helvetica, arial, sans-serif;
        cursor: auto
    }
    #wrapper {
        width: 450px;
        margin: 20px auto;
    }
    h1,h2 {
        text-align: center
    }
    h1 {
        text-transform: uppercase;
        margin: 10px 0;
        line-height: 1;
    }
    h2 {
        text-transform: lowercase;
        margin: 0 0 20px;
        line-height: 1;
        color: #aaa
    }
    ol {
        list-style-position: outside;
        margin-left: -50px;
    }
    li {
        text-align: left;
        background: #ddd;
        padding: 10px;
        font-size: 24px;
        margin: 10px 0;
    }
    p {
        color: #333;
        line-height: 1.5;
    }
    /* the important stuff */
    .cmdDown { cursor: wait }